My thoughts – I’ve mucked around at 50NL HU (clear winner) – 200NL HU (losing/maybe breakeven over a smallish sample, ahem!) and was a mediocre midstakes 6-max semi-regular a couple of years ago.
I found that at 100NL and above you will only get a regular game if you’re a loser. My recent results aren’t that good at either 6m or HU (since I stopped playing regularly), but I’d still get mostly sat out on last time I tried playing, even sometimes at 50NL. (And when I didn’t get sat out on or got joined it was normally a pretty strong player or a short stack)
Also, it seems to be getting worse – so 6 months before that, I could usually get a game at 100NL but not 200NL, 2 months ago usually at 50NL but not 100NL.
Perhaps I’ve been unlucky, or there’s a strong player with a similar name to me or something

(But I think the fundamental problem with cash HU is that there’s no (short term) incentive to play anyone who might be as good as you, unlike 6-max and higher.)
So if you want to play cash (vs SNGs) I’d recommend sticking with the 6-max route (if you prefer shorter handed then try to start tables), and then trying HU once you’re over-bankrolled for 6m at the equivalent limit and can just sit down and wait while playing other games and then not worry if you suffer a big downswing (or encounter somebody much better than you).
I have no SNG experience so I can’t comment on that. You might get some good responses at the 2+2 HU forums, even if some of them are impolite!
If you want to play some tables on here, let me know and I can probably provide some feedback compared to opponents I’ve played in real HU. (I’m sure lots of others here could do that better than me FWIW – I’m mostly just interested in poker theory these days)