I wonder how dramatic it is within latin languages. as you mentioned on macOS you only use latin typesets. For those things most annoying are spelling mess ups, but I mostly use English keyboard and long press when I need to write things like papanaşi (ro) or buñuelos (es). (but I get those red dotted lines).

A lot depends on using a national keyboard. I use ABC - Extended as my default keyboard and turn off long press because I don’t like it, since I’d rather type ş with opt-c s and ñ with opt-n n (I’d say I type maybe 85% English, 14% Spanish and 1% Czech or Slovene), but some people prefer national keyboards for non-English which make diacriticalized letters more ergonomic at the expense of making some punctuation or symbols less accessible (I was just talking with my ex-wife who writes a lot more Spanish than I do about convenient/inconvenient keyboard layout changes—she uses the globe key switching which I avoid because like long press, I don’t like it, especially since I use it often as a fn key and the other functionality often get triggered by mistake until I disabled it).

My pet peeve with Gboard on Android is that long press does not work for Hungarian duuble-acute letters (ő and ű).

It takes some getting used to, e.g. ö used in both Hungarian and German is in two different places for the two languages.

I do ctrl-space a lot, but very rarely use the long press feature, mainly because depending on the active keyboard, the numbers to press are different and it hinders me from being able to type blindly.

Edit: I adore papanași.