I have a 2d fibre connection and run both a 2.4GHz and a 5 GHz network from the FritzBox 7490. Coverage is a bit spotty at one end of the house so I have a second 7490 with a wired connection to the first and running as a fixed IP address client. I thus have 4 wireless networks and have named them all differently so that I can know which one I am connected to (for no particular reason). All have the same key.
Now there is no issue with wandering around the house and transferring between networks as all 4 are remembered by my phone, but we recently acquired a robot vacuum cleaner and it seems that it wants to know only one SSID to connect to.
Obviously, I'm going to try it, but I'm interested to know if there are any hidden pitfalls in running a bunch of networks all with the same SSID and key.