Pretty sure most video cards suck at it. If you really need something with hardware acceleration I'd look at using Quicksync. I'm pretty sure that's by far and away the best option available (for hardware accelerated encoding) provided you're using Haswell or newer. Otherwise nothing beats software still.
Also nvenc can still be accessed through CUDA. The encoder uses dedicated hardware though and not the actual GPU. I suppose if you need something stupid fast and at absurd bitrates then you can make do with nvenc. The new cards coming out soon should have h265 support as well which wouldn't really be possible to do in real time in software so I guess it has that going for it. Just hope the quality doesn't blow like it does on their h264 offerings.
Actual GPU's aren't very good at video encoding.