I have an acoustic bicycle with Gates CDX and Alfine 11 IGH. The use cases for this combo are not for me. The absolute win is the lack of maintenance and no greasy chain. However chain maintenance is a non-issue for me. A little care goes a long way. And I'm familiar enough with bikes to avoid the "chain print."

My issue with Gates drive is that the drag is very noticeable. A properly tensioned belt introduces a lot of drag. Adding in the drag from the IGH seals, I always feel like a brake is dragging. Another concern for Shimano IGHs is that they can leak when the bike is on its side. This is a non-issue for bicycles in use, but my bike is designed to be packed for airline flight. It's almost guaranteed that the hub will be vertical during transport, and I've had to clean up small oil spills after traveling with my bike.

Changing gear range is expensive and always requires a student/new belt. On a chain drive bike, changing gear range is simple and relatively cheap. So if you're going the Gates route, try to have a good understanding of your desired gear range before taking delivery.

Finally, repairing a flat tire on the rear is a way more involved process. If you are very familiar with working on belt/IGH bicycles, it's less of a concern. But changing a tube on the side of the road in the rain in the middle of the night (because OF COURSE that's when flats tend to happen) is a real pain. The change goes from a two-minute operation with a chain drive to about 12 minutes (for me) with the belt. There are ways to mitigate and reduce flats (Schwalbe Marathon tires, tubeless tires, tire strips...), but these all introduce some other factor(sl that either increase maintenance, require more tools/supplies, or increase rolling resistance.

Are Gates drive bad? No. They carry a lot of benefits for urban commuters. People who don't work on their own bicycles are an excellent target audience. The system just isn't for me.