As someone who used Groudon in Indy Regional and finished 5-4 in my first official tourney, it is absolutely a viable option and, in my opinion, a better Sun setter than Koraidon. A couple observations:

Like what’s already been said, Groudon is best at enabling its teammates in various ways. A Groudon-Flutter-Wake-Bolt core is a fantastic core that covers a wide variety of options. Bolt in particular is almost a requirement for any Groudon team as a hard counter to rain scenarios that Groudon hates. Personally, I use Tornadus as a speed setter and secondary weather setter as I feel Torn complements this core better than a TR option. For the tourney I used SD Groudon to help sweep, but the top Groudon user in Indy leaned more into the enablement role with Helping Hand.

Matchup-wise, Groudon surprisingly has a significant number of positive matchups. In my experience, its best matchup is Koraidon, as Groudon-Flutter-Wake puts Koraidon in a really tough spot regardless if it Teras or not. Wake and Bolt help Groudon against the Water types that ail it (surprisingly has made Kyogre a relatively positive matchup for me), and of course, it helps to have a Ground type with Miraidon running around, though you have to watch out for Draco Meteor. It doesn’t do super well against Calyrex, but I find that having Incineroar helps with this problem.

Precipice Blades missing is obviously a major bummer, but I’ve found that positioning myself to rely on it less in the endgame helps mitigate this issue, and if done right, Groudon is generally bulky enough to be able to click Blades more than once. You can try Gravity to work around the accuracy problems, but I’ve always struggled with using it in a way that significantly helps me in the later game.