Imo, deepending on the desired quality of the result and the amount and complexity of bespoke requirements, the more of the former are present, the more strongly I consider rolling something bespoke.

With out-of-the-box libraries, the more custom requirements I have, the more trouble I tend to have supporting them, and trying to make something do a thing it wasn't designed for, can erase initial gains very quickly. At least this has been my experience over the years.

I mean, it really depends. I'm writing models for some energy traders at the moment, and things were simple enough in the mvp that we could hand-roll our optimisations. But when we started having to map real contracts it got out of hand super fast. Linear constraints solvers have been a godsend, it makes adding new types of contacts and constraints really easy (and supports almost arbitrary kinds of optimisations).

It's just never as easy as "don't do this" or "always do this". But yeah, I agree with your gist, right tools for the right job and all that. Most of the time you don't know all requirements up front and it can make sense to hedge your bets in the interest of speed until you do.