It is certainly a good product for NAV capability. And yes, IAN is in my opinion better implemented than the managed variant, although it has a breach in procedure with the inability to fly curved approaches in APProach mode, unlike the managed thing.
However, GLS is available on equipment from both OEMs, airbii are certified for RNP 0.10 approaches as well as boeings are.
But i believe the comments were not about the NAV capability. It was about the more than questionable flight deck ergonomics, the missing EICAS and the ancient overhead panel.