Volcarona is emblematic of everything wrong with Tera: too many viable sets that punish too hard. I think we're all familiar with the experience of bringing in your Volc check (e.g. bulky Cinderace) and then it Teras into one of its 8 viable Tera types and you lose. However, every pokemon does this to an extent. Volc is special because it can win games with so very little setup, but stuff like Iron Hands, Ceruledge, Baxcalibur, Gholdengo, any of our dragons, basically anything that uses setup moves can abuse Tera to get free turns or to blow past checks. To deal with these pokemon reliably, you not only have to have something that can beat their regular form, but also something that can beat them with Tera and setup. For example, Iron Hands is normally checked by Great Tusk, until it Teras Flying and then you're tasked with dealing with +2 Drain Punch and Wild Charge on this Pokemon with gargantuan bulk. On the flip side, stuff like Sandy Shocks and the now-banned Regieleki can abuse Tera to get unresisted coverage. But these Pokemon still have weaknesses: Sandy Shocks falls to anything faster thanks to its frail bulk and Iron Hands gets beaten by strong neutral hits repeatedly. The issue is that you have 6 pokemon on your team that can Tera for no cost. If you are facing a team that isn't favorable to one of your pokemon Teraing and sweeping, then you can just not Tera that pokemon and use it with something else. The other part of Tera that I feel is uncompetitive is that for pokemon like Volc, it's unpredictable which Tera type they are even in-game. Do you bring out your Dondozo and risk falling to Tera Grass or do you bring out your Clodsire and risk falling to Tera Ground, assuming you have both in the first place? Whichever you choose, if you predict wrong, your wall is dead.
Tl;dr- tera too hard to deal with in the teambuilder and unpredictability is dumb in games
Tl;dr- tera too hard to deal with in the teambuilder and unpredictability is dumb in games
