In MySQL, is there any reason not use utf8mb4 ?

If you don't need anything above ASCII, or aren't capturing Unicode data (multiple language support, emojis, etc.), it just takes more storage space... But I generally think the flexibility is good to have and saves having to fix it later when you find out you do need it. (What were you planning to use instead?)

Currently on UTF8 but have been hitting the emoji issue

In that case, definitely make the switch. It is nice not having the constant battle with new Emojis

Lol. They do change a fair bit