In L2.5 bridging, the Client Bridge inserts its MAC address in the source MAC field of any frame that passes through it. This essentially 'hides' the MAC address of the client device from the rest of the network.

Can prevent applications using MAC registration.
In a network where users must authenticate, such as a hotspot location, once one user has authenticated, all users behind that client bridge may be able to use the network.

Traffic originating from behind one client bridge may have intermittent or no communication with devices behind other client bridges on the same wireless network.
If multiple client bridges are used in one 'chain‘, that client bridge then connects via Ethernet to another AP. That AP has one or more client bridges, network communications in either direction may be intermittent or non-existent.

See below:

This is an ARP request. It’s destination layer 2 address is the entire broadcast domain and it’s layer two source address is it’s wired NIC card. As you can see it knows the destination’s IP address (layer 3), but not the 192.168.1.90’s MAC address.

Below is a reply to the request.

Download arp-nat.pdf for more info.