I used Swiftkey until recently, but the delay on bringing up the keyboard was annoying me too much. I couldn't justify changing phones just because of SwiftKey so instead I just went back to the default Google keyboard.
I'm still adapting weeks after changing, the most annoying issue is that the Japanese keyboard on Gboard has the key layout a bit different that the English one, so I cannot touch type on it like I did before.
The features that kept me on swift key is that is multilanguage, Google keyboard you have to switch to the correct language beforehand. I'm pretty worried that SwiftKey is now doing big nonos like sending my data around, but Google keyboard is absolutely unfit for my usage.
Gboard does have multi-language now. In fact, I think it's better at that, simply because SwiftKey obsessively capitalizes the word i, even if you try to delete that suggestion. That's one of the main reasons I recently switched away from it, plus the fact that if you put the cursor in the middle of a word SwiftKey moves out to the end, and the fact that SwiftKey frequently hangs for several seconds in the work profile.
Google Keyboard is multilanguage too, if you mean having multiple languages active at the same time and the swiping/hints responding to both. I'm currently typing this comment using it.
I'm still adapting weeks after changing, the most annoying issue is that the Japanese keyboard on Gboard has the key layout a bit different that the English one, so I cannot touch type on it like I did before.