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Incarnate is easy mode for ALL FG farmers because he wastes card slots of Retros. Every turn he draws a Retro, you get one turn to set up your board. Even RoL/Hope beats it with ease, while on paper Retro > RoLs.
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Only reason why Incarnate is so easy for RoL/Hope is that it eventually fills its board with useless Skeletons and you know that yourself. Retrovirus being bad card, which it isn't, has nothing to do with it.
Thats not really the only reason, but yes its a part of the reason. Retros actually "help" filling board with skellies. But
eventually doesnt bother most FG grinders, as after you get your setup down, you should be able to win. Fact is, Incarnate is slow as hell to get his damage down, and that is the reason why most often than not, you'll be able to draw your setup. Its not only RoL/Hope, its control rainbow decks or doll decks too.
Having all creatures poisoned is a lovely effect. But it comes with a HUGE price: card slot, turn to wait, and damage received until poison actually kills its target.
Fractal Retros with current Soul Catcher works. But thats due to Fractal that allows you to both nullify card slot price, trigger tons of death effects, and gain enough quanta to follow by fractal dragon.
Value of Virus/Retrovirus as a card itself is easily seen in usage of those cards in PvP events.