You pretty much nailed it since Nadus was usually either a rain-setter, people would generally anticipate that and proceed to maim him so that he would not have much of a chance to abuse hurricane but on times he did it wasn't too difficult to wall it. Or Nadus was used as a BU, I'd recall normally seeing this outside of rain teams, and personally found this to be somewhat more potent as it is more difficult to wall or stop since STAB acrobatics flight gem really hurts especially with a boost or two (and if it carries sub even tougher). Though in both cases, mostly in the rain dance set, he is capable of setting up tailwind before he dies that gains momentum for the team.
I'll be honest here I never had much luck pulling off Meloetta's mixed set or CM set as well as I did with just a simple Relic Song + 3 attacks. Though I did have more issues with mixed and CM sets because Meloetta was quite easily killed in OU, lots of dangerous scarved revengers or mons that just plain hit hard (don't need to explain those), since it did not have much speed and psychic typing is too easily dealt with in OU the story would probably be different in UU. Still one thing about the CM set is that Meloetta lacks a recovery move, couple this with lack of resistances and of course being vulnerable to all forms of hazards, it was easily worn down.
Now as for Relic Song + 3 attacks I just adore this set, it requires hazard support and is best used late game when you have an SR+Spike on the field or at the very least have eliminated bulky mons, its ability to change speed tiers as well as gain a very potent STAB attack in CC makes it much more difficult to stop or revenge. I find its best not to abuse CC too much and rely more on her other moves unless you're certain that no remaining priority attack will take advantage of the defense drops. Another thing to keep in mind with this set is Relic Song, don't just mindlessly transform but ideally look for something that relic song can actually hurt that becomes easily cleaned off by Pirouette form. Most of the time I try to set up on those weaker on the SpD side so that Relic Song can do quite some damage. Its also helpful to remember the types of attacks the mon you're setting up with Relic Song as it won't be recommendable to set up on something that is much slower than you which hits Pirouette form hard with a super effective attack or a strong special attack. Last important thing to remember with Relic Song + 3 attacks is be sure to kill off anything that might have carried TW/WoW and Intimidate (especially intimidate since it would generally force you out since a lot of things will easily survive your STAB CC even with hazard support).
Still what I especially love about this set is the way Meloetta tends to attracts dark or bug types that end up being smashed so badly by a fully attack invested Pirouette form that gives you much ease in eliminating them. As well as screwing over speed tiers and changing its set of counters with a single move. I find Relic Song + 3 attacks to be very balanced and I feel can managed in UU, well no surprise here since the tier is quite prepared for fighters, as it only has one STAB that is covered easily (I can't help but wish it had extreme speed as it would have saved my ass so many times with the things Quick Attack failed by a margin to clean up). Meaning you've got the choices of Stone Edge (accuracy issues), Acrobatics (loss of item), Ice Punch, Quick Attack (wish this was Extreme Speed), or Shadow Claw (forget this in UU ghosts are bulky here and only frail ones are covered by a stronger neutral attack). I find Meloetta does well against HO teams, she can easily tank a hit and transform into Pirouette form and proceed to outspeed and KO the frail sweepers, but does have more difficulty against stall and balanced. Frankly, Relic Song + 3 attacks Meloetta is lot like Kingdra in that respect in that it does well against more offensively oriented teams but does have difficulties with more defensively oriented ones. Again I am only speaking of Relic Song + 3 attacks I cannot say how the other two sets will go.
