What happens when you mix an RPG and a shooter to make a game like Borderlands 2? Nick Dinicola at Pop Matters wasn't too fond of the love child:
At the risk of sounding misanthropic: I hated it. In my experience, groups of two or more tended to play Borderlands 2 as a shooter, while I still wanted to play it as an RPG. Suddenly the contrasts between these genres became obvious and detrimental. Borderlands 2 was no longer a shooter-RPG hybrid, but a shooter impeding on my RPG.
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He says the styles clash, but the biggest problem here is not the action of the shooter, but the inherent problems with co-op:
It’s also hard to explore an environment when your team (even if it’s just one member) is focused on completing objectives. When one person runs ahead, everyone else begins to miss out on the actual content of the mission. Borderlands 2 is still part shooter, so if you’re not shooting when someone else is, you’re missing part of the game. My desire to see every aspect of a mission forces me to play catch-up with whichever player is moving the fastest. Even when a partner is simply helping me find hidden symbols or complete a collectible challenge, he either demands I follow him as he runs straight to the objective or he’ll just do it all himself. Instead of pushing through the game at my own pace, I’m being pulled through by someone else against my will. Such a relentless pace is another shooter staple, but RPGs should be slow and methodical. It results in the odd and discomforting feeling of being dragged through my own game.
An RPG is hard to play co-op when two players have different goals on their minds. World of Warcraft is not at all an action game, but when you want to read some lore or converse with a quest giver, by the time you set out, your teammate who skipped everything could be halfway through the quest, which is the same problem Dinicola faced. The way around this is to be on the same page when playing any RPG with another person. The sub-genre will not affect the outcome.