Fetlar's not that difficult. Of the outer Shetland Isles, it's pretty reasonable.
Out Skerries is short (381m) with unlit obstacles intruding into the approach path and no lateral run-off area. At night using flarepots with strong crosswinds it concentrates the mind wonderfully.
Fair Isle is much longer & wider but has a truly vicious downwind turbulence if the wind is greater than ~20kts from Sheep Rock. Also from an area off hills to the north. I've managed to get in when the wind was gusting 30kts & variable from Sheep Rock but it took several tries to to each runway to pick where the turbulence was then an unusually steep approach with lots of gust margin with a short, sharp flare & landing.
Foula is worse! If the wind is >15 kts from the West or East then the strip is all but unuseable although I've managed it on rare ambulance occasions using lots of local familiarity and throwing stabilised approaches out the window.
Even my base airstrip at Tingwall could be challenging in a 45 kt x-wind component at night onto an iced or partly iced strip in minimum visibility up the Tingwall valley.
I doubt that these are the worst in the UK but they're certainly not conducive to complacency.