In a recent update to the HSBC app they’ve added a screen to prevent you from using the app unless you use the default (google) keyboard.
They do a similar thing if you have an accessibility service running that can access the screens content. A fair enough security warning if you’ve happened to install a dodgy keyboard app, but highly frustrating when using an open source alternative that enhances the security and privacy over the default option (HeliBoard in my case).
I haven’t found a way to circumvent the page yet. It would be useful if Android allowed you to block the permission to query all packages, but alas.
As someone who doesn’t have the time, skill, or knowledge to audit open source projects, I agree on the trusting more popular open source keyboards (and by extension popular open source projects in general).