Well, that just makes it worse! OP strongly implies that there is no good variable-speed solution, but if the industry already has an assortment of products, what exactly are they even selling?!

I think you're misreading the OP slightly. There's no cross-compatible system for variable equipment, which is why the OP says you can't use variable speed equipment with Nest. Nest is basically a really fancy switch only. To use variable speed equipment, you have to use your manufacturer's proprietary system. I'm not positive this is right, but I'm pretty sure it's backed up by https://www.pickhvac.com/central-air-conditioner/extras/comm...

So basically, the premise of this product is that the proprietary system they design will be nicer than the proprietary systems HVAC manufacturer's design. That's probably true, but as an owner of a Carrier proprietary system, it's totally fine.

The issue isn't that an open standard for fully variable speed isn't possible, the issue is that nearly all of the manufacturers to date have considered this proprietary (as they want to own the entire ecosystem from controls > equipment) and a purposefully boxing Nest, Ecobee, etc, out of being able to control it in order to sell more thermostats and trying to capture more value.

Only problem, their thermostats and apps really suck.

Heat pumps are basically ACs with inverters allowing them to work at partial load (i.e. 10% pr 20% or 70%). Otherwise, they are as efficient as ACs at full load.

There are few off the shelf AC thermostats that work with most heatpumps: they are usually the on/off variety.

Remotes coming with heatpumps would do the right thing, though, and there are IR blasters that can replicate those signals and integrate into your home automation system.

I think you have to go thru a licensed HVAC installer to get an inverter system. These guys are DTC.