There is absolutely no problem with bridging and VPLS to transport PPPoE.
you just need to do it right
i concentrate hundreds of VPLS in a central location on one single bridge. Even using VLANs there is no issue at all.
To prevent bad things from happen I just enabled horizon-bridging all the way down from the PPPoE-Access conentrator to the CPE.
This ensures that each customer can only see the PPPoE-AC.
There is no Layer2 connectivity at all between the customers.
Also absolutely no firewall rules are needed, thus enabling for faster processing within RouterOS.