Maybe it has a poor implementation.

aspell works great. Back in the day I used some IBM employee written software on DOS that was a TSR that would spell check words for you in popular editors. In the 1990s every word processor had a decent spell checker. They all had the ability to add your own additional words.

Agreed, aspell is noticeably better than many of the others I've used over the years.

I suppose poor implementations of boring old deterministic spell-checking are a thing, too.

Maybe that's some part of the disconnects in this thread.