I've always had issues with third party keyboard support on iOS. They always were more glitchy and had issues after using them for a while. The iOS keyboard has always provided a better experience. This is in contrast to my experiences on Android where third party keyboards were ALWAYS better than the stock options. I don't know what it is about the implementation on iOS that makes third party keyboards worse (in my experience). Maybe they are better now. I haven't used one in over a year now. My last one was GBoard. Which I LOVED when I first started using it. Then I started having issues with it not displaying when I wanted to type or responsiveness not being adequate. So, as I always do, I went back to the iOS keyboard.
3rd party keyboards have improved greatly. I love using SwiftKey. It's not glitchy at all and has a much better memory than the stock keyboard. I use SwiftKey as my default keyboard and keep the stock keyboard deleted. SwiftKey recently added 3D Touch to control the cursor.