Ok so I've had a problem with tentacruel for a long time, and maybe I'm a little biased but i think Tenta is just fine
Staying in A3.
Tenta is a sort of mixed bag in a way and while it offers a lot of role compression and both offensive or defensive pressure as Panther aptly discussed, my real problem is that it just doesnt quite fit into any of its roles.
What I mean by it not fitting is that its stats betray it in certain ways, as a defensive mon its not incredibly bulky and its base 65 defense really hurts it, as it's forced to fully invest in defense and doesnt really check things on the defensive or specially defensive side, forcing you to run another water type or fire resist lest you lose to Entei, and another fighting check so that you dont lose to conkeldurr or any other really hard hitting fighting type, and another fairy resist so that specs Sylveon doesnt blow you away and etc. Its typing is another mixed bag, while offering some nice resistances it also gives a crippling weakness to ground, one of the most common coverage options in the tier.
On the offensive side, again it's a mixed bag with a base 80 spA that leaves a lot to be desired. This spA stat forces it to run Hydro pump which is a risky move and it unfortunately misses out on alot of crucial KO's even with max spA investment and hydro pump. In addition it faces a pretty big case 4 moveslot syndrome, forcing you to choose between ice beam toxic spikes haze and sludge wave.
While these points are all well and good I do still believe tentacruel is a very good mon in the tier, but looking at the mons in A3,
Swampert which is one of the best physical walls in the tier for balance and offers stealth rocks and roar with a strong offensive presence,
Crobat which is one of the best fighting checks in the tier with defog, taunt, u-turn, one of the best speed stats in the tier, and coveted flying stab,
reuniclus one of the most reliable defensive set up sweepers in tier,
Infernape which destroys standard balance cores and constantly keeps the opponent guessing for its set,
nidoqueen which hits incredibly hard, offers nice utility with rocks and T-spikes, and is fat as hell, and
Mamoswine AKA Mr. No Switch-Ins, Tenta fits pretty comfortably here.
Looking at the tier above it and one of its biggest competitors (and imo the actual best hazard removal in the tier)
Empoleon is, in this meta at least, far and away more reliable than Tentacruel. Its defensive set, though sharing a ground weakness and gaining an ugly fighting weakness, has an actual defense stat allowing it to fully invest in Sp. Def making it a premier Sp. Def wall and providing crucial resistances to Rock Dragon and Flying (one of like 2 flying resists in the tier) and nice utility in stealth rocks and Roar. Its offensive set also sports an amazing 111 spA stat which helps it so much as it gains a whole lot more offensive presence, and its defense stat allows it to run Shuca which nabs all sorts of surprise KO's on mons like Swampert and Krookodile. Finally Defog in general is much better in Klefki meta seeing how it can't just be blocked. Even Tornadus which punishes Defog has its stab resisted by Empo, so you can play around it seeing as its forced to Superpower/Focus Blast.
In sum while Tentacruel is a fantastic mon in the tier I just don't think its quite up to snuff with the rest of A2 and fits perfectly well where it is, thanks to its unreliability, poor stat distribution, 4 moveslot syndrome, and inferiority to Empoleon. Feel free to disagree I just wanted to voice concern on my behalf, and thank you for reading n_n.