LOL! Dear god, that is just going to tear apart EVERYTHING. Let's do some damage calcs.
CBDuggy (Adamant Nature, full attack EVs) vs standard wall Steelix: 426 Atk vs 436 Def & 354 HP (100 Base Power): 212 - 252 (59.89% - 71.19%). Guaranteed 2HKO. There goes Swellow's #1 counter.
Correction: Steelix is guaranteed to
only be 2HKOed, whereas thanks to Arena Trap and basic human logic, Steelix will never actually be switching into an Earthquake and will always be able to launch at least one attack. Let us see how much that attack does, shall we?
Standard Smogon Steelix (Relaxed/Impish, 120 ATK EVs) Earthquake vs CBDugtrio (4 HP/0 DEF):
88.68% - 104.72%
That is almost guaranteed to be a OHKO with entry hazards down, and even has a chance to be without it. ...Of course, the aforementioned Swellow + Dugtrio tactic is not very likely to come in on an Earthquake, so how about a different attack?
Gyro Ball:
106.60% - 125.00% (Calculated at 120 BP, results may vary)
Stone Edge:
29.72% - 34.91%
Even a non-STAB, resisted attack on the switch does a fair amount of damage. So, if anything, I would say that Steelix "counters" Dugtrio, not the other way around.
Here are some other calculations I ran, just to experiment:
Standard Technitop Mach Punch (Adamant, 252 EVs) vs. CB Dugtrio (4 HP/0 DEF):
93.40% - 109.91% (Guaranteed OHKO with hazard or Fake Out and >50% chance to OHKO otherwise)
And, as for the best priority that Dugtrio has:
CB Dugtrio (Adamant, 252 EVs) Sucker Punch vs. Standard Technitop (200 HP EVs, for the sake of argument):
18.90% - 21.99%
Standard Careful Registeel (100 ATK EVs) Iron Head vs. CB Dugtrio:
63.68% - 75.00%
CB Dugtrio Earthquake vs Standard Careful Registeel (252 HP/0 DEF):
76.10% - 89.56%
Both are 2HKOs.
As Dark_Azelf said, Dugtrio itself is not really an offensive power as much as it is a revenge killer, but it can only really switch into opponents that it can safely KO in one attack, due to its poor defenses. So, the most that I really see Dugtrio doing in UU is serving as a revenge killer for certain Fire- and Poison-types. ...Although...
+0 Toxicroak w/ Life Orb (Adamant, 252 EVs) Sucker Punch vs CB Dugtrio:
89.15% - 104.72%
...Not even that is for certain.
On the other hand, saying that Dugtrio will help deal with stall teams is not exactly true, either. Because it kills some of those Poison-types, that cuts down on opposing Pokemon to absorb Toxic Spikes, which in turn might promote their use a bit more by giving UU stall another way to keep them on the field. So, if anything, Dugtrio might be
promoting stall usage, in a way. Of course, this is naturally all theorymon, so feel free to ignore it, but I will be interested to see what kind of effect this actually has.
.....That last part sounded far better when I first thought of it...