Thank you very much - thats an excellent answer!While I wouldn't put too much weight in the current rankings (there are plenty we'll be changing hopefully in the near future), OU Viability does not translate 1 to 1 with BSS Viability. There are plenty of mons that are strong in OU but mediocre in BSS, or worse. The reverse is also true of course, Mimikyu and Celesteela are pretty broken here but OU doesn't seem to like those two as much as we do. Stuff like Snorlax and Lapras also tend to be way better here than in Smogon metas, though that leans into other stuff like items that are stronger here too (Assault Vest, Sitrus Berry, in the case of other Pokemon there's also the coveted Focus Sash slot).
Sheer Force is a dead ability slot with Dynamax and its Max Move options are pretty mediocre for a special attacker of its kind (no Special Airstream option is pretty bad), and Landorus-Therian is far superior which further increases the opportunity cost of using Landorus-I in the first place. It's not exactly a terrible Pokemon or anything, there's just not a lot of reason to recommend it at the moment over other faster special attackers (ex. Nihilego), and of course there's better special Airstreamers like Kantonian Zapdos and Galarian Moltres.
For future reference though, feel free to ask future viability questions in the VR thread itself. Don't be afraid to question something there!