-- knock yourself out
Everything I am about to say relates purely to ladder play, since I am not a tournament player and will not pretend to have in-depth understanding of tournament planning.
Tera Blast is problematic because it's high variance, and specifically, it's
unpredictable variance.
Competitive gameplay requires a degree of predictability - Clefable has thrived for generations, thanks in large part to having a movepool containing virtually every move it could desire, allowing it to adapt to a changing metagame despite unimpressive stats, and having the tools to make use of both of its excellent abilities. Regieleki was UU in Gen 8 due to its poor coverage options, and when Tera Blast solved that problem, it was immediately yeeted to Ubers.
Tera Blast is often compared to Hidden Power, and it's a valid comparison, but the two differences - the degree of commitment and the raw power - mean it serves an entirely different role. Hidden Power had three main use cases: as a STAB move to offset a poor movepool (Gyarados with Hidden Power Flying), to snipe a 4x weakness (Hidden Power Grass to snipe Swampert, Hidden Power Ice to snipe Lando-T), and the occasional use to hit a target that resisted everything else the user could bring. With a maximum of 60/70 BP, it's not strong enough to be more than that; it's a lousy STAB move and - generally - poor coverage; neutral STAB would routinely beat out super effective Hidden Power.
Tera Blast is stronger. 80 BP STAB is an effective 120 BP, or literally twice as strong as Gen 6-7 Hidden Power, enough to be a powerful coverage move. It's also very committal, either you teratsalize the mon carrying Tera Blast, or it's a dead move slot; the only mon that gets anything out of an 80 BP Normal move is Regieleki, and that's solely because it's the best move to hit ground types.
Thus, Tera Blast falls into two categories: valuable coverage move, such as Serperior and Regieleki, or a lure, such as Kingambit sniping Fighting types with Tera Blast Flying or Fairy. The former is unproblematic because you can reasonably expect it, and the mon ends up evaluated by judging the extra coverage versus the commitment required to access it - Volcarona was deemed broken, Serperior fell to UU. The latter is the problem.
It's not just that it's a lure; rather, it's a lure that
everything has access to, and is powerful enough that - unlike Hidden Power - it's not relying on targeting a 4x weakness to pay off. Somewhat ironically, if Tera Blast was a better move baseline, if it wasn't an un-move on most physical attackers and lousy on special attackers, it would be more common and less of a problem. You'd expect it, you'd account for it both during team building and during play, and keeping tools in check to deal with a less-surprising Tera Blast would be just another hallmark of skill.
Physical Tera Blast on Kingambit wouldn't be an independently good move, by any means, but you could run it on a SD/Sucker/Kowtow/TB set and click the move to hit Dark resists and not be completely hosed when needing to terastalize something other than Kingambit. In lower tiers, you could run it on Gyarados for that Water/Normal neutral coverage, and turn it into a STAB Flying move if useful to sweep; again, not a good move, but a usable one that offers some value at all times.
By becoming more common, and less of a pure lure, it'd be more predictable, and thus less of a tiering challenge; Kingambit having a move to nuke Great Tusk would be a normal (albeit uncommon) thing, rather than a rare tech that cannot be reasonably expected. As it is, SURPRISE TERA BLAST! isn't something that can be predicted, isn't something that can be reasonably played around, and thus isn't - by the actual Smogon definition of the term - competitive.