First of all, I've only recently started to play RU during the last couple of months, so I never experienced the haven of BW RU that others have mentioned; I primarily played OU before, but RU piqued my interest. With that out the way, I want to say that I've enjoyed this tier a lot more, and that I have been surprised by the variety of playstyles that I've seen - balance, HO, and stall have all been used and viable, with different teams used successfully on the high ladder during the suspect test from the matches I played and the replays I saw.
I think all of the three suspects are tiltering either side of borderline broken. The team I used might have been prepared for them mind, though opposing Mega Birds should have raped me: I used LO HW Mesprit/LO Dugtrio with rocks, Standard Serp, Work-Up Mega Bird, Reckless Emboar, Unburden Hitmonlee, and SD Doublade.
Dragalge
Dragalge for me was the one that troubled me least; it's been mentioned that it has a great match-up against offense, but it has so many checks that its manageable. Offensive teams are used to sacking 'mons anyway and don't rely on counters; I could sack a 'mon and then either trap with Dug, or revenge with Hitmonlee. Even though its special bulk is pretty good, its physical bulk and speed lets it down, making it easily revenged. Even Emboar 2hkos the standard specs set with unboosted flare blitz:
252+ Atk Reckless Emboar Flare Blitz vs. 200 HP / 0 Def Dragalge: 135-160 (42 - 49.8%) -- 91% chance to 2HKO after Stealth Rock
Despite this, it's not the Specs set that makes it broken. I think the variety of successful sets that it brings to the table tips it over the edge. I agree with numerous posters that have stated that the Specs set is easily taken advantage of by bringing a Steel and a Fairy type, forcing it to avoid spamming its stabs (this is easily acheivable for defensive and offensive teams - eg. Bronzong+Aromatisse / Cobalion + Slurpuff), or setting up on it at -2. The plate sets and T Spikes sets are where it's really dangerous, especially as the Specs set is more common and the element of surprise comes into play. Given that I packed Dugtrio on my team, it was also less of a threat than it could have been.
Mega Bird
I'm really torn on Pidgeot to be honest. I used two sets: the standard Hurricane, Heatwave, U-turn, Roost/Defog; and Work-Up/Refresh/Hurricane/Roost. Both sets were underwhelming and overwhelming in different scenarios. I often found it to be the least useful member in my team to be honest - so many teams were prepared for it with AV Eelektross or powerful scarfers that it was easily worn down, and while powerful, it's not quite powerful enough to break through defensive teams with Cresselia, Registeel, and Rhyperior. It's bulk is also not that impressive, with priority attacks and faster scarfers able to revenge it with just a little prior damage.
Some posters have also mentioned that Bird + Dugtrio could be broken, but I found that Dug just doesn't do enough to Bird's biggest counters:
252 Atk Life Orb Dugtrio Earthquake vs. 212 HP / 252 Def Solid Rock Rhyperior: 152-181 (35.8 - 42.6%) -- 93.1% chance to 3HKO after Leftovers recovery
252 Atk Life Orb Dugtrio Earthquake vs. 248 HP / 0 Def Solid Rock Rhyperior: 183-218 (42.2 - 50.3%) -- guaranteed 3HKO after Leftovers recovery (destroyed in turn by EQ in both cases)
Nevertheless, the confusion chance can be gamebreaking. I found myself staying in a lot of the time on a Kabutops that had come in to tank a hit to get the 3HKO, knowing the odds were pretty reasonable to get confusion hax at least once on the two hits. This is where it becomes uncompetitive, and where I can see a ban being viable.
Serperior
A free 130BP Nasty Plot sounds really tasty and it is, but I found the ladder was really prepared for Serp. Every defensive team had Amoongus and/or Golbat, while every offensive team had at least 1 mon faster than it, as well as a scarfer to revenge it or priority in the form of Mega Obama or Mega Face. With weaknesses in the form of low bulk, susceptibility to all hazards, and both Leaf Storm's low pp and 90% accuracy, Serp just didn't blow the ladder away.
Nevertheless, the pressure it causes opponent teams both offensive and defensive alike is incredible; you can't let it setup, so it's counters and checks are brought in right away, making them easier to wear down. I can definitely see why a ban is desirable.
Finally, I think Dragalge and Serp should get the boot, but I'm not sure about Pidgeot.
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On another topic, I also think that it would be better to test the subjects in isolation, as testing them together can affect the results of the test. For example, a major check to Serperior is Mega Pidgeot, so Serp might be deemed as not broken due to its high quantity of checks, but when Pidgeot is taken out, Serp is seen as broken and has to be tested again (hypothetically).
Things like Pidgeot/Scarf Moltres aren't ideal answers to Serperior. If you hard switch one of those into Serperior, you risk getting hit by Glare, after which setting up a Serp sweep is fairly trivial.
Pidgeot/Scarf Moltres should come in on and successfully revenge/force Serp out, due to their position on offensive teams, who don't mind sacking a mon. The problem is the limited times they can due so due to rocks.
Sorry, but why would you use Muk or Probopass in RU? Probopass is merely a check to Dragalge, as it carries Focus Blast and Scald on most of its sets; its defensive typing is flat-out garbage on top of it. Though Muk may check much of the S-rank, I fail to see what it accomplishes outside of that.
Also, (and here my lack of depth of knowledge of the lower tiers could be my downfall) I don't like the way that people discount niche counters and use them to explain how broken a 'mon is, i.e. Probopass + Muk. If Muk checks much of the S-Rank, then surely it becomes a viable 'mon in RU? Surely this is just the fluidity, innovation, and creativity of the meta coming into play?
Damn I just realised that is a LONG wall of text. Sorry!