In competitive, is there any reason to use Return over Frustration or vice versa? I know that using Frustration can help against Imposter Dittos, because they usually run the default max friendship. Are there any other reasons? If there is a reason in one Generation and not in another because of mechanics changes, please point that out too.
Thank you.
Practically I would say there are few implications. However there are implications with respect to Transform / Imposter since the transform/imposter user will retain their own friendship value but gain Return/Frustration from the mon they copy. From an game theory exploitative strategy standpoint, since Return + 255 Friendship is the default, one ought to run 255 friendship on Ditto/ Transform users and 0 friendship + Frustration on your attackers in most metagames.
Of course, the nash equilbrium is probably closer to 50/50 mix on 0/255. In theory, you could even imagine some metagame where Dittos ought to run some intermediate value to not be fully walled by a setup sweeper running the opposite friendship level and mono self coverage (e.g. a dnite with return/frustration + eq coverage).
source So these are the only official competitive battling mechanics that are affected by friendship/happiness. There might be other mechanics in weird Smogon formats, like gen 8 National Pokedex back in 2020.