Because you'd be buying a product which, while it could potentially do the job standalone at layer 2, wouldn't be as well-integrated as another AiMesh router, like an AC66U_B1 for similar cost. Either that, or hardware-disable wifi on the 68U and purchase two working-pull Ruckus R500's off eBay for $50-55 each and run them Unleashed. Either way, you're giving your endpoints a way more uniform link-layer experience off of either AP, plus a single control plane, both of which will give you better seamless roaming and easier management. And not much more, if any more, money spent.
True. I've only a few sites so my experience is very limited. I'm also a novice so I lean towards presenting a view with the novice (me) in mind.
Even though there are standards and vendors do try to adhere to the standards there's always room for error so, you're right, there's always the possibility of interoperability issues between multiple vendors.
But, on the flip side, you can take a
great router yet when you dumb it down to be an access point it might not be so great. Same with AImesh, I'm sure in the near future it will full fill the dream but, for now, many report it's not quite there yet.
I've heard great things about Eero mesh (vs. Orbi) and even the Google mesh Wifi as long as the OP is looking at mesh anyway but, myself, if I'm only looking to add strength in one (versus many) areas I simply add an AP.
I've extended two of my sites with Netgear 6150 range extenders. One is hardwired. The other is wireless, I used "fast lane" so one radio acts as a dedicated backhaul to the router and the 2nd is dedicated to our clients. So they're both "layer 2-ish" and they've both been running great for a few years now. I do not seem to be having any interoperability problems between vendors.
At another site I've two Ruckus APs connected to an Asus RT-86U. (You're smarter than me, I had to pay a few hundred each for mine.) They are great APs. Reportedly the easiest PRO device to configure but, that said, there is still a bit of a learning curve. My favorite feature is I can easily isolate "guests" from my private LAN. I did, however, leave my Asus acting as a 3rd wireless access point. Again, no apparent interoperability problems.
I've walked all three sites over and over and, as yet, have had no problems with hand offs. Although we do use VoIP we have no portable VoIP devices so I can not speak for how well those types of hand offs might fare.
Anywho, for backyard use, maybe an outdoor theater, most any AP should meet the OPs needs.
There are many good choices out there, hopefully it will be difficult to make a "wrong mistake".