Happy to help!
For me on an Xbox 360 controller, I find resting my index fingers on the bumpers constantly is a tad uncomfortable since it's a bit of an extra stretch compared to the triggers. I've heard on PS controllers that it's more comfortable. Also someone told me that they sometimes use their middle fingers on the triggers with their index fingers on the bumpers, which I find to be more comfortable but I've never played a game like that (I always use only my index fingers) so it seems foreign to me (though interesting). I don't know if any of this applies to you, or maybe your tendons just stretch further than mine or something. :)
I think for me the bigger issues with the control layout were: 1) using triggers/bumpers for melee attacks rather than face buttons seems a bit odd, just because in my experience it's much more common to find melee attacks mapped to face buttons so that's what I've become accustomed to over the years, and 2) having to take my fingers off the movement controls (triggers) to access the attack controls (bumpers) is a real issue, although if you were someone with that 4 finger grip I mentioned above that may not be an issue. Still, I am sure there are many who like me only use their index fingers.
Rebindable controls is definitely a must. :) But even with them I always like to encourage devs to choose their defaults carefully since that's going to be most people's first experience with the game and especially for a game such as this with complex and unconventional controls since many players may not know what will work for them from the beginning.