I stand to be corrected but from what I have read, the UK ETA is similar to ETIAS which I now understand will operate in a similar way to ETA and will be compatible with EU e-gates as in the UK.

The EES system is completely different and the UK doesn't have an equivalent. Obviously EES is covering a very wide area ie the Schengen zone and it will check for people overstaying prior to passport control and also calculate how much time they have left in their 90 day allowance. Once these checks have been completed, they can then proceed to the EU e-gates.

Im not suggesting what you say is wrong, in fact having seen Spanish airports with kiosks of various shapes sitting around waiting to be installed you are probably correct, its just that other countries manage to roll multiple systems into single e-gates - residency, visitors on e-visa, vistors on long term visas, equivalent of EES etc all processed through single gates. Typically you have a one time visit to an immigration desk on first visit to scan your biometrics after that you stroll through e-gates.

I guess also in the EU the exact setup will depend on the infrastructure of each airport, as some dont have a huge amount of space and some dont have e-gates at all.

I take your point but I don't want to be stuck behind someone at an e-gate who is struggling with the EES system or trying to cheat the system by overstaying. It's better for them to be rejected prior to passport control otherwise it will cause delays for everyone else.

It seems sensible to hive the EES system off separately and before passport control.

I've been to lots of airports where there are massive amounts of booths for manual passport control... but I've never seen more than 20-25% of them active at once, even with massive lines. It's like with these large supermarkets (hypermarkets)... so many checkout counters yet ALWAYS so many of them closed, even during the busiest times of the year.

They'll find the space, one way or another... Though I'd hate to be around if the system ever fails and suddenly EVERYTHING needs to go old-school, via human hands...

Time will tell but having travelled through dozens of airports with e-gates typically anyone who doesn't meet the criteria gets a big red X on the screen and funnelled off to an immigration officer without anyone else being inconvenienced, usually the only delays are caused by people putting their passport in the wrong way or not looking at the camera etc the length of the delay usually depends on how many 'guides' the airport employ to smooth the process.

Whatever happens it can only be a good thing (I hope)

Yes .... either manual booths are closed and frequently numerous e-gates closed. It always amazes me arriving at Heathrow when it seems like a shock to them that 3 aircraft arrive with 300 passengers on each as if they (immigration) weren't expecting them.

Exactly! I've seen some really bad lines at Heathrow though luckily not during recent travels. But it amazes me when you see those closed booths/gates with massive amounts of passengers piling in... as if those planes suddenly landed out of nowhere and everything was completely unexpected.