This is on PS5/NA, but I'm sure it's a similar situation for the other platforms.
Outside of events, nobody does them and the rewards one gets from a Harrowstorm is not near lucrative enough to match the effort one has to put in. You essentially get the same rewards as a chest and maybe some vendor junk that comes out to possibly 500 gold. The Western Skyrim ones are essentially dead at all times and if there's players there's usually only a couple of them which makes doing them a slog. The ones in the Reach are still somewhat active as long as you're in prime time, but again very hard to find people outside of that.
I am a good enough player to solo them but that takes 20 minutes in a proper setup for a reward that I can get from a base game dolmen that takes maybe 2 minutes to do, on a level 10 character with basic gear. Some of the furnishing plans are expensive but they're also very rare, I did all the harrowstorm achievements and maybe got 2 blue plans that were worth little, leading back to my first point.
I get the feeling ZOS is not eager to touch these as it's now older content and if they were they probably would have by now, considering this has been a problem since launch. Scaling like from Dolmens or the Summerset geysers should have been included from the start, methinks.
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