I think the real highlight is that it is not backwards compatible. So I now have an MX Keys and an MX Master 3s, both on their own dongle.

IIUIC, Unifying receiver used a completely different protocol, but Bolt protocol is just supercharged Bluetooth with some low latency and encryption sauces on top.

The old unifying receiver was also able to pair up to 6 devices.

Sounds nice, except so far I have only one Logitech device that works with the bolt receiver, which is the MX Master 3s.

I also have an MX vertical which uses the unifying receiver. Then I also have a pro x superlight, which has yet another receiver. I believe they call that one lightspeed or something.

So now I have 3 separate adapters for 3 devices, all of which theoretically support pairing multiple devices (I think the lightspeed only does 3 though).

Doesn't bluetooth handle many, many more devices?

It also introduces a ton of lag, especially when more than a single device and some distance is involved (Mouse + Keyboard + Headphones makes my headphones cut out sometimes)

Bluetooth MIDI keyboard + Bluetooth headset = audio glitches. Disappointing.