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

Long press on the iPhone used to (maybe still does?) omit ý on the long press for y and I think some other uncommon Czech diacriticals and I had to add a Czech keyboard to type those, but the other changes around that keyboard were really annoying and I would occasionally switch to it inadvertently causing weird autocorrect issues.