MIMO works fine for me. The jump from 802.11g to 802.11n with two or more spatial streams was a significant bump in speed. And yes, I mean speed as in TCP, not PHY rate.
Yes and no. You can always go faster with more TX power, but if your neighbor can do the same then you get interference.
I can see 20 networks around my house. If everyone 20xs their power it would probably be thousands competing on a few narrow bands.
Curious why you say this. In the last few years, I've seen a significant increase in PHY data rate when using multiple spatial streams.
The snakeoil comment was about the MIMO functionality which I can only really test via my iPhone7 at the moment with the 2x2 MIMO client to the 4x4 MIMO AP (UAP-AC-HD). Once I get the MacBook Pro I'll have a 3x3 MIMO client.
I bought primarily because if this animation explaining the benefits of MIMO. https://unifi-hd.ubnt.com/
The marketing worked on me!
There's no magic bullet, and buying a bigger more expensive AP is, on its own, likely to make no real difference to any wifi benchmarks you might throw at it.
However, what you do get is the ability to easily place several APs in your house, know they will work together seamlessly, as you would expect from enterprise-y gear, and end with a more reliable and consistent wifi network.
(The article goes into detail why raw speed won't be gained by buying the bigger AP, so I won't repeat it here...)
Honestly the big reason mobile radios are more reliable is that they're allowed to transmit at like 20x the power of devices in the ISM band. That's not a cost thing, it's just a regulatory issue steming from (some rather outdated) concerns about interference in the shared environment.
I mean, it's true that wifi is sort of a mess. Marketing of the standards has gotten way ahead of real world hardware capability (frankly MIMO as it stands is basically voodoo snakeoil that provides no consumer benefit whatsoever). But it's no less complicated than all the junk going on in LTE either. Everything's a mess.
Wifi continues to have the extraordinary advantages of being privately deployable, performant, pervasive, and standard (i.e. ethernet-framed, so everything Just Works the way everyone expects). It's not going anywhere.