Took a chance this evening and messed with binding my Aqara hub to HA. Some disappointments, but it's not all bad.
I totally forgot that HA doesn't create entities from stateless devices like button controllers. Since this driver needs an HA entity to create a Generic Component device in HE, buttons will not work. For most, this probably isn't a big deal, and for me it's not huge since most of my buttons are Lutron Picos. But it presented a problem for my 6 button Opple which is a favorite of mine. Sure, I've heard others have had good success with joining it directly, but where does that leave the Mijia button in my son's room? That thing drops pretty quickly when joined directly to HE, but never drops when joined via the Aqara hub. He likes it and I have a few more of those unused at the moment, so the Aqara hub needs to stay. Besides, that serves as my doorbell too!
On the plus side, if you do bind an Aqara Gateway to HomeKit Controller in HA, you still get control over the alarm and the light, including color control. RGB color control from HE of a device joined to HA might be too much to tackle though, since HA use CIE-XY color mapping. However, the color controls are still there in HA, so you can at least pass it forward to HomeKit for control from the Apple Home app.
Personally I'm now thinking I need to keep both my Aqara Hub and Mijia Mulitmode Gateway active. The Mijia Gateway will serve to bring Aqara and Mijia contact, motion and temp/humidity/pressure sensors into HE, and the Aqara Hub will stay bound to Apple HomeKit via my Apple TV 4. The Aqara hub exposes the Opple button controller to HomeKit, but for some reason, the Mijia Gateway doesn't, so the Aqara is best suited for buttons in the official HomeKit.
In the end, I've actually increased the number of active gateways in my house by one . The Mijia Multimode Gateway has been sitting plugged in, but not doing anything for a year anyway, so at least it's finally useful. But overall I realized as I was rebuilding my required HomeKit automations, what a nuisance it is to have to configure them, and what an annoyance it is that, as soon as a device disappears from HomeKit, so too do the HomeKit automations. I'm therefore very much looking forward to this change.