I mean technically if they were chatty for some reason that would take up air time, which is limited.
Or if you were getting close to a maximum number of Z-Wave devices that are supported, obviously that would be an issue.
If you have a C8 (or C7 with antenna mod) what you're more likely to find is that the extenders are sitting there not doing anything, and not very many things are actually routing through them.
Doesn't really hurt anything, except your wallet.
Edit: While not necessarily a good or bad thing, I have seen Z-Wave routes be a little schizophrenic when you have a lot of routing devices. Lots of route changes for no real good reason when they have 30 or 40 different routes they can take. Without doing a repair, or a packet being missed, the routes are supposed to be sticky. But my experience is that in large zwave networks, the routes tend to change more frequently for no obvious reason.
When I had the C7 hub, i needed several Z-wave range extenders to connect to devices such as ones in the garage and in the basement. The lock on the front door was also finicky as there is a large HVAC return duct between the hub and the lock.
Once I upgrades to the C8 hub, nearly everything is connected directly with the hub, including the front door lock.