I operate 12 adapters in 3 different phases and have no losses.

The lighting network in the flats is very rarely divided into several phases, it must be a huge object - so there should be no such problems

With different phases, the Power Lan adapters usually work quite poorly. You will certainly have a large transmission loss if they work at all. But it also depends on how long the cables are until they come together at the fuse box or central distributor. I have tried it with older Devolo 85Mbit adapters and got a maximum distance of about 30m with different phases. But this depends very much on the house installation.

Difficult to say. I think there are always losses. But the things are getting better and better; no comparison to a few years ago. It's best to try them out and return them if necessary.