Using the PKCS11 certificates on a Belgian eID is cumbersome because the whole 'client certificate' workflow has always been a pain in the butt UX-wise. Many workarounds have been implemented to improve on this, making it less platform-independent than theoretically possible.
The itsme app works totally fine for me, and my less tech-literate family members. It strikes a good balance in UX and security.
All in all, I think that the Belgian government is doing a good job in this. It needs to work well for the average user, which it does! I don't expect my government to spend time and money to support all fringe cases like smartphones that don't run iOS or Android.