Thats the thing! There is no problem. Muun uses a different system to pay for the lightning invoices than the standard, this system is convenient, but might have problems of scalation. So, if in the future it starts to have high fees or other problems, just stop using it, maybe Phoenix for other reasons starts having problems and if that happens you just change to other.

It Is the beauty of this system is easy and free to change. The important thing is to not be marry to any of them.