Normally that’s fine because these are very low power networks which are designed for infrequent messaging. It’s quite common in Europe to have them in apartment buildings. Typically the walls between apartments are more like exterior building walls then normal interior walls, and that in itself tends to help block signal a little bit.
If you are running super heavy message traffic, either because of polling or because of energy reporting, you might occasionally run into some interference but in that case the message should be resent, so it might just mean that things run a little slower but the messages should still get through.
Quite a few households, including my own, run multiple Z wave networks with different hubs without any problem. I’ve had as many as five running at one time, and now I’m down to three. No noticeable lag or interference.
I’m not saying it’s impossible that there would be interference from time to time, but it’s not usually a problem.