A setup can be smoother and more intuitive to the point of creating a more enjoyable experience by trimming the 2-7 redundant buttons of each combo step having its own key without that "making or breaking" the setup as a whole. There are more and finer thresholds than just "broken" and "workable".
If we're to call not even having the option to forgo button bloat a worthwhile contribution to "mechanical difficulty", then where does that end?
The same would then apply to anything, even just lack of polish. If the way anything gives something more to manage (to do or not do, however unfun it may be), then that would excuse its cost to polish or other gameplay options.
- This skill is bugged and does less potency that is stated? Well, those who know are rewarded, so that's an extra measure of skill-gap and therefore beneficial.
This skill crashes the game? Well, like all but your 1-2 GCDs in combo sequence, don't press it. Those who don't get rewarded. Surely that's worth the extra button.
There are gameplay reasons to keep the option of having each step even of a singular, inflexible decision require its own button, but "mechanical difficulty" ain't it.
Tactile feel? Sure. But
pure traps aren't worth spending buttons on.