It was unusable. Because of the constant size changes you would hit the wrong keys and some were getting so small you couldn't hit them anymore.
Really the only downside is having to put a bit of effort into learning it, but for the amount of time I personally spend typing on the phone over the years, it's well worth it.
I also tried other keyboard formats. 8pen was kind of interesting, but I could never get it very consistent myself, it felt more like a gimmick.
[0]: http://www.8pen.com/
I'll second the recommendation.
Indeed, I noticed that if I disable auto-correct then keep using very simple words, and short messages, I have much less problems.
Of course, answering with emojis mean no risk of typos.
Often I'm trying to type in names of wines, domaines, vineyards, etc. - the CellarTracker app is probably the destination of the majority of my keystrokes. I could swear sometimes that the wrong character got entered even though I was pretty sure I hit the right key.
And of course now I'll be hypersensitive to this possibility, and cherry-pick evidence to confirm my suspicions.
[0] https://www.cnet.com/news/iphone-keyboard-learns-what-you-ty...
[1] https://www.microsoft.com/en-us/research/wp-content/uploads/...
[2] https://static.googleusercontent.com/media/research.google.c...