I initially had unbelievably terrible experiences with Opus 5 and Fable in their higher reasoning levels.

I've had WAY better results on medium effort.

IIUC, the consensus seems to be that anything more than medium effort is rarely worth it - and you far more often run into these extreme worst cases than you do with even the lowest effort levels. That definitely coincides with my anecdata.

It's really only worth it if you're hoping to win the lottery asking it to solve an Erdos problem.