We can use Steam in home streaming as a yard-stick here.
That is, one PC playing the game, and another sat in front of the gamer.
I have a Steam-link, and while it's indistinguishable from a full PC, lets use a second PC as the client machine, so we can benefit from having the Steam-link's limits removed (e.g. it only has 100mbs wired networking).
In this setup, we have two PCs, both connected to the internet network via wifi, but connected together with a 1GBs Ethernet connection, which can be forced into use for the gaming stream.
This is essentially the best possible setup for streaming a game across a network - if a single cable can be considered a network.
I haven't done any particular in-depth tests with this, aiming only to satisfy my desire to play casual games from the sofa, older games were chosen to reduce the workload so as to not bottleneck the encoding and streaming:
Picture quality - pretty good overall. At times, you can notice artefacting, which appears similar to turning the sharpness up too high on a TV, or jpg compression on images.
Delivery quality - Over a direct wired gigabit connection, the stream is rock-solid, with maybe a little hiccup maybe once an hour.
My current setup, with a 100mbs steam-link connected to PC through two switches is just as solid.
Over Wifi, it's a different story - even on 5Ghz uncontested link, hiccups occur, at a rate of 2-3 a minute, lasting a second or two each time, which is't terrible from a performance state, but is immersion breaking. Reducing the image quality reduces the frequency and duration of these hiccups, but they still occur and you have a much more noticeably compressed image.
Delivery latency - One of the games I tested was Mirror's Edge (2008) Constant latency is distinctly noticeable, even with the direct wired connection. It's perfectly possible to play the game, but you have to anticipate all actions. Most notable is when performing a roll when landing - playing locally, you pull the trigger just before hitting the ground - playing remotely, you have to pull the trigger prior to landing - when about 2m in the air. The same latency is noticeable when engaging in combat, making shooting harder (smooth movements are required) and disarms of AR armed enemies pretty much requires slow-mo due to the short period given to perform the move. I have a fairly sensitive NFS Shift configuration set-up with almost no dead-zone on my wheel. The latency made racing effectively impossible, being almost impossible NOT to over-correct for even minor deviations from the racing line. It was like trying to drive in one of those drunk-driving simulators.
From memory, the reported latency was reported by steam-link as about 25-30ms but I'd take that with a grain of salt. I'm pretty sure there are additional sources of latency that are not being accounted for in this value, and this is a local connection. Over the web, I would expect latency to be twice that.
For "casual" games, it's great - and I'm using casual in terms of "not requiring reflexes" here - games I can relax when playing. Something like XCOM-2 works great, but local streaming on anything requiring fast reactions can be anywhere from having annoying latency to unplayable.
Lets just say that I'm not optimistic about web game-streaming providing an experience that suits anything other than "casual" gameplay.
That sort of game can typically survive being run on lower powered (aka cheaper) hardware locally, albeit at "cinematic" frame rates without causing issues to the gamer. This is where I think the competition will be, and the cost equation is a lot different between a £300 machine and a £1200+ machine when compared to the costs of an online gaming subscription.