As I understand it, you can learn everything 20% faster, but they should only be mediocre, but they can not do everything perfectly. For example, there are some stems that can not handle some things because of their biome and body size, so these tribes specialize in a few professions, making them legendary in the profession and making Neran a master or grandmaster. It may be that even a Neran can become legendary but that will probably be more difficult in the end and will ultimately take longer.

I've translated that with Google Translate for spelling mistakes I apologize.

I think that is one of those details that have yet to be fleshed out. Since we don't know yet how long it takes to learn a skill (or if different skills have different learning times), we can only try to influence the design. For one, a 20% bonus to learning overall would be OP. Just to learn a new skill though to the next level, would be fine.