Followers have a hard-limit on their CHC and CHD and going above those values provides them with no additional damage. Once they’re at those values, it’s more valuable to add a Ruby to their weapon, to increase the base damage, but it’s all pretty much irrelevant because I have an Enchantress with 48K INT (all gear slots ancient with 625 INT augments on them) and she can’t kill mobs in Torment 16. Followers are purely there for the buffs they provide to you, i.e. think of them as mobile Kanai’s Cube powers.
As for legendary gems, Esoteric Alteration and Mutilation Guard will work on followers. However, this is only useful / required if you’re planning on not using the follower’s “cannot die” token, and you’re building them with lots of survivability so that you can take one of the other tokens, i.e. the “access all abilities” or the “50% cooldown” tokens.
These aren’t bugs though. As per the stickied How to Write a Good Bug Report
Feedback is NOT a bug . Anything related to feedback, class balance, boss suggestions, etc. belongs in another forum.
Don’t use bug report threads as a platform for discussing game balance, design, or to provide feedback or other subjective opinions about the game.