They're stable, it's just that some of the visual cues in a low hover are missing.
If you sit at 1000ft and just look out, you'll find it hard to tell if you're descending or not so you have to include a scan of the VSI as part of your instrument check.
Likewise as you slow from forward speed to nil
(a) you lose translational lift so you need more power
(b) the need for large pedal inputs can sometimes catch you out the first few times and the machine suddenly swings on them as they counter (a) and find that they suddenly get affected by weather-cocking.
And the wind direction and velocity at altitude isn't the same as it is on the deck either.