Latency. Sadly, Ethernet over powerline (IEEE 1901) made the same unfortunate choice as WiFi and the (analog phone line) modem V.4x standards in including error correction. This is not what you want or need for TCP/IP. It's better to drop a packet than to deliver it late. IEEE 1901 tries to mitigate this by using forward error correction, but succeeds in that only to a degree.
Bandwidth is alright-ish at about estimated 2MiB/s here (single pair, about 50ft apart), but RTT of ~4ms with occasional spikes in excess of 1s (at a different location, I've seen spikes in excess of 5s). This is only barely noticeable for accessing the WWW, but unpleasant for interactive GUI work (e.g. via NX or VNC) and a deal-breaker for cluster communication protocols.
Bandwidth is alright-ish at about estimated 2MiB/s here (single pair, about 50ft apart), but RTT of ~4ms with occasional spikes in excess of 1s (at a different location, I've seen spikes in excess of 5s). This is only barely noticeable for accessing the WWW, but unpleasant for interactive GUI work (e.g. via NX or VNC) and a deal-breaker for cluster communication protocols.