The real problem I had with RE5 was that everything was so tedious to complete. Enemies just take forever to die, but it's not like it was challenging, everything just became a long drawn out process of running back and forth getting a hit every now and then. Killing with the knife became a standard in RE4 once I mastered the game, in RE5 it just feels like a waste of time considering how long a simple enemy takes to die. The most fun part for me was The Mercenaries.
I still liked it and completed it a bunch of times in both co-op and single player, but it was probably due to me thinking RE4 is the best game ever.