Started playing this in No Hope Left in co-op recently (beat it a few years ago but never got the chance to play the hardest mode with someone). Game is definitely pretty fantastic despite being unpolished compared to Resident Evil 4 and 5; I might actually like it more than 5 overall. Honestly, I think there are two big problems with the game, putting aside the lack of polish and awkward non-standard bits - 1) despite being pretty complex mechanically, the game doesn't really explain your abilities and their uses well at all (nor does it put you in situations that organically teach you the strengths and weaknesses of your weapons and such, which is understandable given the game's complexity and the multi-campaign system) and 2) the difficulty modes seem poorly balanced and are mostly too easy. Normal is mindless, Veteran is a little easy (seems to be a slight step down in difficulty from even RE5's standard mode), Professional is what you'd expect the regular mode to be like, and the only mode over that is the super-hard mode that most people probably never touch. And most of the game's modes being too easy is a problem because without a reasonable degree of difficulty, most players aren't going to bother experimenting with their toolsets and learning all the cool options they have in interaction with enemies, so most people are going to just mash their way through the game not understanding how interesting its combat scenarios can get. Not saying RE6 doesn't have other problems, or that everyone should be able to overlook those two issues like I personally do, but if the game was smarter / a little less forgiving about its difficulty setup and did a little more to expose its options to the player (probably should have forced the player through a comprehensive Vanquish-like tutorial at the start of the game), it may have been received much, much better.
Like, imagine if Resident Evil 4 had three difficulty modes - one named Normal (let's say it's an easier mode than the easy settings that appeared in later RE4 releases), one named Ultra Hard (let's say it's identical to RE4's normal mode) and one named European Extreme Death (identical to RE4's Professional). This is basically what happened with RE6 - most people who play the game are going to pick the "standard" mode, which feels kneecapped since combat can only be so engaging or interesting when the enemies can't really damage you and die to a stiff breeze. On top of that, pretend this imaginary version of RE4 has no tutorials at all, which might make the melee system opaque to a lot of people - this version of the game might not have been so beloved back in 2005.
I definitely don't agree that Mercenaries is massively better than the campaign - No Mercy is definitely a fantastic supplement to the main game, as RE5's Mercenaries was, but regular RE6 Mercenaries is kind of sparse. The individual fights in RE6's campaigns are generally very good, even in some of the game's worse levels like Leon-1, and the level design is actually really good about handling co-op (does a better job of incentivizing or forcing players to split up than RE5 did).
I really ended up liking Resident Evil 7, but it's definitely disappointing that Capcom didn't follow up RE6 with one of their patented Iterative Capcom Sequels - the game probably deserved a second pass more than even RE4, honestly.