I love my iphone. It has excellent stabilization and I have uploaded a lot of iphone clips.
Otherwise I prefer a real camera, but my xsmax is really amazing.
iPhone is definitely acceptable, I've used mine a lot, and all the shots have always been accepted and most of them sold. The trick is to know your way around a video editing software, and some basics colour grading skills will help too.
Bear in mind its limitations thou, like you can't have a one size fits all DSLR camera and a lens that can shoot in every situation, phone videos are usually really bad in low light settings.
So for stock websites, any of the new generation phones will do as long as you clean up the shots in post. It's best to get a DSLR if you just want to upload straight from camera.
All in all, quality doesn't matter as much these days as prices are going down, as long as you produce 'acceptable' quality according to the websites standards. It's the content of the shot that sells.