I think if one plans to use kstars/ekos on something like a Raspberry PI and try to do all your imaging via VNC in a remote session, then I agree the Pi is woefully under-powered. But that's not how it should ideally be used and the INDI platform's client/server architecture makes the Pi more than adequate. As you noted, the best way to run it is by letting the client do all the heavy lifting. If someone wants to only use one device and access via VNC, then a device more powerful than the Pi would be a must.
It's not the only issue of the Pi that the CPU isn't powerful enough. There's also the bottlenecks of (no) USB3 and the network. I have several Pis, but for INDI I (currently) use an older ASRock Atom unit, and that although I do run EKOS on my laptop. But the gain you get from USB3 (at least with a cam like the ASI1600) and a real Gbit ethernet speed for remote desktop sessions is priceless (IMHO). And the sufficient CPU power allows to easily run PHD2 on there, too, with a fast camera connection and the option to run more demanding algorithms at high frame rate.
I learned early on that the most annoying part of this set up is the file transfer "latency" when a module (solver, focuser, capture) needs to capture an image. It really impacts things when you are doing an imaging run and saving all the images to the client. That can introduce 30+ seconds of transfer time between subs. My solution is to do all my pre-work (eg. plate-solving, focusing) as normal, but when I fire up an imaging run, I change the Upload parameter to "Local". I have a USB thumb drive adapter that fits my 64gb microSD card and I plug that into the Pi. The next morning, I pop it out and process the files on my Mac.
Yes, that's exactly what I meant above. With my setup download of a full 16MP image of the camera to the client takes a second. A complete plate solve (2s exposure, download to laptop, plate solve locally) takes around 5-6s.
Don't get me wrong - I think the Pi (and even more Stellarmate) is an easy and quite cheap way to test this type of setup. On the longer run you definitely want something better(*).
Now, just in the past two days, I added one more piece of equipment that is a game changer for my set up. I was constantly struggling with reliable wifi, so I bought a pair of powerline ethernet adapters. These things sound too good to be true, but they really work amazingly well. The units plug into wall sockets; one of them near your main router and connected to it via ethernet, the second one is plugged into any other socket in your house where you want a fast connection. The units basically turn your electrical wiring into an ethernet network. I ran a heavy duty extension cord out to my scope, plugged in the second powerline adapter and connected it via ethernet to the Pi. Freakin fast! Now when I take a preview in Ekos, it loads on my Mac in 12-15 seconds (it used to be 30-40).
Ha, maybe I should consider this, too. ATM I'm putting two cables out, power and ethernet
(*)I also just ordered an ASUS PN40 as replacement for my ASRock. Not because of too little power, but for size. That is indeed something where the Pi is really good. But with 12x12x6cm the PN40 isn't that much larger....