ye its also do do with things like water flow and"decor" if you will.
plecs like fast flowing water withlotsa wood and even rocks where as discus are happiest in gentle flowing waters with lots of upright folage like val and swords and the likes
im not saying they shouldnt be kept together otherwise i would be a hipocrite
but just be mindful of things other than just water parameters.
and lornek8 unfortunately i have had one attack ONCE of a discus by a peppermint a few years ago.
i was on holidays and my housemate was feeding my fish for me. in one of my displays she forgot to feed and veg or even algae disks for the full 6 weeks. although not fatal the plec did at some point take a munch on the side of the disk but that was extreme circumstances.
other than that i have never had any problems with them together
their not nasty just got the apetite of a vegeterian teenager!!! ha ha ha