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In my opinion, Dugtrio is UU because it would get destroyed by half of those in the OU tier.Pokemon such as Metagross, Scizor and Tyranitar, some of most common pokemon, "lol" at the likes sucker punch and stone edge and threaten to OHKO back (if its choice banded. If it isn't, they "lol" even more) The same applies to Weavile (to a lesser extent due to it being more fragile.

It also loses to the like of Swampert, Zapdos (physically defensive), Mamoswine, Bulky gyrados (offensive gyrados is primarily a different story but that's besides the point), Bulky dragonite, flygon, skarmory, cresselia, hippowdon, jirachi, suicune, Lucario, and other pokemon I'm too lazy to list.

This is assuming it's adamant. If it was jolly (the only other viabale nature for it seeing as the remaing stats suck a**), they, like metagross, scizor, and tyranitar, would ROFL seeing as now dugtrio is even weaker.

It could take some, such as infernape and starmie, due to arena trap and sucker punch but almost half the rest of the tier could take it down quite easily due their bulk and dugtrio's lackluster DEF/SDEF (for some of those, even more so with a resistance to its moves).

In addition, it only resists rock and poison moves, both of which are uncommon in OU (and UBER for that matter). Moreover, with it fragility, it may not even survive the attack even if the attack in question was one of those two types.

One last thing, u-turn and baton pass users such as ambipom and vaporeon can escape arena trap, making him even more useless against the likes of those two plus umbreon (if it has baton pass), leafeon, and other batonpass/u-turn users.