"unique ability" so does that mean infected drone should be epic as well? Or splitter bot? Unending drone? or like pretty much anything? Sure some epics are unplayable but if they are played, they generally have big stats or do really powerful things. This guy just doesnt really do anything. Trades weak stats, easily removed, high cost for very specific ability trigger, two lives, gets slightly stronger not epic material at all.
The choice of wording might have been bad here. What I meant is that higher rarity cards tend to have more complex abilities, rather than just being better. Tygris, the Architect is a perfect example of that. It's a terrible card and it's a legendary. It's ability is quite complex though and that's why it deserves a high rarity.
When I look at various cards at different rarities this is what I notice most. Epics aren't better in general. In fact, a lot of them are outright garbage. Yes there are those that are very good and they stand out, but when you look at all of them you can't really say they are better on average.
These are the things that I would say correlate with rarity:
-Complexity: There are some exceptions but there is a noticeable trend here for higher rarity cards to have more complex abilities.
-A stronger divide between good and bad cards: Legendaries in particular tend to be either great or awful. Commons tend to be meh, ok or good. This may be a consequence of higher rarity cards being more complex.
-Higher rarity cards more often need to have a deck built around them: While this is certainly not true for all epics and legendaries it does apply to a number of them. Probably a consequence of both the first point and the next one.
-Higher purity requirements. Obviously the Agent Coyles and the titans have an impact here but even if we take them out of the picture this trend is still obvious. There is the occasional factionless card though.
-Unique characters: Most noticeable for commons. There is only one unique common in the game at this point(Serennia, Champion of Life, which would probably have been at least an uncommon had it been designed as a champion from the start. Tithe Collector is no longer unique.). The dragons are the main exception to this.
Higher stats and higher cost seems to be something that applies to epics in particular. Legendaries tend to be more reasonably costed. There are examples of similar cost epics though. Ireul, Nix, both versions of Nysrugh and some others. Nix is amazing, but some of the others seem similar in power level to Silfurstar if not worse.
I don't believe that rarity=power in Infinity Wars. Just look at all the amazing commons and uncommons. Splitter Robot is arguably one of the best 4 cost cards in the game and it's only Uncommon. The 1-drop wonders are all common. Even Wealthy Noble, which probably decides more games than any other card, is a common.
I don't think there's anything wrong with Silfurstar being epic at all.