I have AT&T U-Verse, and they use IP multicast (and IGMPv3) for IPTV. Receivers wired into my network work just fine but receivers connected over powerline Ethernet don't. Computers connected over the same powerline link work just fine.
These powerline Ethernet adapters are sold as "bridges", but they don't actually appear to be bridging. A bridge should forward all broadcast/multicast traffic unless it can determine that such forwarding is not necessary, shouldn't it?
Yes, they work like a bridge, but this doesn't mean they will work with IP multicast.
There are different types of network bridges. Only the simplest bridges forward all traffic to every port other than the source. Most network bridges are "learning" bridges and operate on the data link layer. They inspect packets and maintain a filtering table with addresses and ports.
Keep in mind, powerline adapters aren't just used in pairs--you can have multiple adapters networked together. Most consumer powerline adapters use the HomePlug AV or AV2 specification. These specifications include features such as QoS for video streaming and AES for security. These aren't simple devices.
