Powerline speeds here at short range are close to the maximum possible speed from a 10/100 Fast Ethernet port. But extend the range slightly and the speed drops considerably.
This is a general problem with power line networking technology. Signal degradation from household appliances drops performance for power lines in the same way longer distances do for wireless. And likewise, results vary depending on the location you're testing the hardware. So Asus has not specifically performed badly here as other devices may not fare any better in our test environment.
Compared with what's possible from a standalone router, these wireless speeds aren't amazing either. With 2.4Ghz 802.11n from a high-end router, you should expect speeds above 100 Mbit/sec, and much better range performance too.
Arguably, you might not be using the PL-N12 at long range anyway. If you need wireless in a specific room, it's likely you'll install the adapter in that room, rather than sit some distance away. And if you need better wireless, you could connect a router in bridge mode to the Ethernet port of the PL-N12, although this is an extreme set up scenario.