My fingers naturally rest on the back buttons, and I find them really easy to use. I usually have them set to mirror the face buttons for shooters (so I don't have to take my finger off of the right stick to jump/reload/etc). In some other games I have them set as dpad buttons (Elden ring for example) to swap gear and use items.

Finally I don't like button mashing, so in games where you have to mash a button I will often set the back buttons to be turbo buttons, where I can hold to spam a key.