I have one, and I sure wouldn't recommend it beyond being a simple display driver. The real killer on that card in particular is that it, so far as I can tell, does not have any (?) video encoding/decoding acceleration whatsoever, and generally the support on AMD cards is mediocre at best until you step up to very high power cards.
That's actually one of the painful parts about video on Linux, is that if you've got something that needs GPU acceleration via libav, you've got three (or four) not so nice choices: nvidia, which has good hardware support but is hit or miss if it actually works outside of their proprietary nvenc, especially for libav and vaapi; AMD, which seems to lack hardware for this on low power cards, or it's poor where it exists; Intel, where the ARC cards don't work well (or at all) if you lack resizable BAR (which means you can't use anything with any age as the host). Intel's integrated graphics is probably the best choice, but still I'd really like a discrete GPU with good Linux h264, vp9 and av1 encoding and decoding support, as it doesn't seem to exist today.
The 6400 lacks video encoders, but it still has decoders for H.264 and H.265. VP9 decoding should also work, but it's not listed on AMD's website per se. To use these capabilities on Linux, you will need to install the correct VA-API drivers, though, and Firefox is still reluctant to enable hardware video decoding on Linux.
The resizable BAR problem is annoying, but that's not limited to AMD, either. I'm not 100% sure about Intel, but if resizable BAR doesn't affect performance, that only makes me think they left some easy performance win on the table.
I have to agree on Intel QuickSync, though, that's hands down the best video encode/decode solution for consumers. AV1 encoders are very recent additions to the GPU landscape, though, so you won't be able to get those on a budget. AV1 decoders are easier to obtain, and H.264/VP9 especially so.
