I've spent a lot of time playing Merfolk in Modern, and I am currently playing GW Value Company with a blue splash just for this card. I believe this card is much better than others think, and I want to explain why.
The main restriction of Unified Will is having creatures in play, so I will be talking about counterspells in the context of creature-based decks.
Why run a counterspell in the sideboard at all?
The right counterspell can be a huge tempo swing. With a creature based deck, it can mean winning the game on the spot when you counter the opponent's sweeper or other expensive spell. In many cases, the main deck configuration includes some kind of creature interaction which can be a dead card in other matchups. Countermagic also gives you flexibility in how you play the game. Holding up the counter on turn 2 to stop a key piece while deploying threats as you make land drops, or flooding the board with threats then untapping and holding up the countermagic. Depending on the matchup or draw or who plays first, you can shift between these plans in a way other sideboard cards don't allow. Countermagic also can be a way to outplay a weaker opponent, or protect your best draws when you need to get lucky to win.
The main reason I play Unified Will in the sideboard is because of flexibility. In general you would rather have a stronger card in the sideboard and the more flexible card in the main deck. However in Modern there are so many different decks it is impossible to have the right sideboard card for every matchup. Painting with a broad brush, you have creature-based decks, control decks, combo decks, and midrange decks. A card like Path to Exile is good against creature and midrange decks, where as a counterspell is good against control and combo decks. Swapping your removal for a counterspell after sideboarding is a very easy start to having a good sideboarding plan.
Which counterspell should be in the sideboard?
Since flexibility is the big selling point, you want the counterspell that counters the most spells (that you care about) from your opponent. In Modern, some of the possible options for counterspells include:
Unified Will
Negate
Disdainful Stroke
Deprive
Ceremonius Rejection
Delay
Spell Queller
Remand
Mana Leak
Flashfreeze
As a creature deck, the cards you want to counter are different than if you were playing a control deck. Creature removal, especially mass removal sweepers, is something you have an interest in countering. Trading card for card and two mana for one on a Lightning Bolt or Fatal Push isn't the most efficient play, but sometimes it is enough to win the game and that is all that matters. Countering an Oblivion Stone or Anger of the Gods is much more likely to win the game. Because you are interested in countering these types of spells, Disdainful Stroke doesn't quite do what you want to do.
The other class of spells that you want to counter are combo cards that kill you before you can kill them. Ad Nauseam, Scapeshift, Gifts Ungiven, or even Ugin can fall into this category. Negate does a great job at countering all of these, perhaps better than Unified Will. However sometimes the card that kills you is a creature. That can be Primeval Titan from a Valakut player, or Wurmcoil Engine, Ulamog, or a large Walking Ballista from a Tron player. Since Tron and Valakut are two of the more popular big mana decks, it is important to not fold to the other half of these decks.
Ceremonius Rejection and Flashfreeze are only good against a few of these decks, not all of them. Remand and Mana Leak only are good in the early game, and sometimes not even enough then. Deprive requires two blue mana, which is fine if you are playing Merfolk, until you want to cast both a Lord and a Deprive with three Islands and a Mutavault (not to mention Cavern of Souls!)
Delay is interesting because in many ways its the same card as Unified Will. If you have a significant army in play, it gives you 2-3 attacks to kill them. If you don't have any creatures in play, you will probably still lose, like if you had Unified Will and couldn't cast it. However Unified Will lets you counter the important spell, and still play Magic the rest of the game. Sometimes you kill them in the next 2-3 turns, but sometimes you need the game to go much longer to win. In a deck with a lot of creature-lands like Affinity, Delay might be better since your army isn't always "on the battlefield".
Spell Queller counters Supreme Verdict, while none of the other counters can. However, it counters so few of the other spells you care about I don't think its worth comparing to Unified Will. It could be an option in addition to Unified Will though. The one mana counterspells like Swan Song, Dispel, and Spell Pierce can be a good option but do not have the reliable flexibility of Unified Will, but are good enough you can play them in addition to Unified Will.
What are the drawbacks to Unified Will?
I think the above discussion shows how Unified Will is the most flexible and probably best of the countermagic options, but what are its drawbacks compared to the others, or compared to not playing a counterspell at all?
First drawback - you need blue mana. In a deck like GW value, you already have 7-8 blue sources with Noble Hierarch and Birds of Paradise. Adding 1 or 2 shocklands into a fetchland mana base gives you more than enough. However if you are already three other colors, like Abzan or Naya decks, the cost to adding blue might be too great. Also your blue mana needs to be able to cast non-creature spells. so decks like Humans with all the Cavern of Souls variants can't take advantage. This is a big reason why I usually avoid playing Cavern when I play Merfolk. Playing a hate creature or artifact is much easier on the mana.
Second drawback - Its not a threat. Drawing multiple counters and no creatures means you aren't putting any pressure on your opponent. It's also not a creature for Collected Company. I think you can play the games you draw multiples in a way to still take advantage of them, but it sometimes means countering a spell you wouldn't normally care about. When you sideboard them in while playing a CoCo deck, keep in mind your total creature count. It is okay to sideboard out some Companies if they aren't going to be amazing!
Third drawback - you have to hold up mana. In a deck with a large amount of instants, holding up counterspells isn't an issue. That isn't the case for most creature decks, who have to use mana on the main phase to deploy the threats. Threading the needle between holding up mana and pressuring your opponent is hard to do but not impossible. Patience can be key against slower control decks, and against fast combo decks a risk/reward analysis will be helpful. The more expensive the counterspell you are trying to hold up, the harder this is, so a cheaper spell can be preferred.
Fourth drawback - you need creatures. Getting creatures onto the battlefield should be your goal anyway as a creature deck, so that isn't the issue. Having more than your opponent in these matchups is usually not a problem, but it does mean you can't use Unified Will vs other creature decks where you might want a counterspell or two (countering All is Dust or opposing Collected Companies or Searing Blaze). But the situations where you have no creatures in the matchups you want Unified Will, you were probably going to lose those games anyway.
What do I bring Unified Will in against?
Just a list of decks I would bring it in against. Think about if any of the other counters would come in as frequently or do as much.
Tron
Valakut
Ad Nauseam
Lantern
KCI Combo
Mill
UWx Control
Grixis Control
Storm
RW Prison
Living End
Grishoalbrand
Time Walk Deck
8 Rack
The biggest benefit to running Unified Will is having so much SB coverage in just 4 slots. This lets you use the rest of the slots to focus on actual or perceived bad matchups, what you think might be popular in the metagame, or just playing cards you really like. I'll hopefully talk more about sideboard construction in the future.
Thanks,
Ty
