Here's why I don't think Naganadel is broken
1. It's frail. Obviously frailty isn't a good excuse by itself, but Naganadel has a typing that provides it with little set up opportunities compared to something like Pheromosa, for instance, Pheromosa had the ability to set up on the most common Pokemon in the tier (Defensive Lando-T), and was literally impossible to revenge kill Speed-wise, your only bets were your own +Spe Scarf Pheromosa, or priority assuming it wasn't paired with Lele. Naganadel is slower and obviously, it has its own slew of viable Pokemon that are capable of revenge killing it and pressuring it at +1 Spe. A list of some includes Scarf Greninja, which now has new life breathed into its niche instead of just a mediocre scarfer people slapped onto teams for Volc insurance, Granted there's only 2 scarfers (Koko is very borderline for revenging Naga anyway), theres still other methods of Speed control, including Rain, Sand Drill, Hawlucha + ANY TAPU (this one is particularly huge because its so damn threatening to offense and it shuts down so many threats too). There's also more than enough priority moves in the tier that will chip it very quickly and pressure it, this is a lot more obvious when you add in SR, potential Spikes and Sand. To list some of these moves: Mega Lopunny with Fake Out and Quick Attack, Mega Medicham with Fake Out and Bullet Punch, Mega Pinsir with Quick Attack, Mega Scizor with Bullet Punch, Weavile and Mamoswine with Ice Shard, Weavile also has Pursuit to completely remove Naga from play if needed. Zygarde with Extreme Speed, Bisharp with Sucker Punch. All of these moves go a long way in showing how how frail Naga is and how easily it can be picked off thanks to its frailty.
2. It provides a well needed addition for offense teams, as it currently stands, offense teams have no good Toxic Spike removers, as they will either lose to Toxapex (Gengar and Scolipede) or generally have no room on offense (Venusaur and Amoonguss), or are constantly getting ran into the dirt by metagame trends (Gengar again and Tapu Fini). Fini's not an absorber, but it usually provides /decent/ countermeasures against Pex thanks to its Terrain and Defog. As it currently stands, the tier has banned too many Pokemon that beat Toxapex, increasingly making it harder, if not NEAR IMPOSSIBLE to punish use of it, and it doesn't seem like a Toxapex ban is on the councils slate, despite nearly all of them hating it and agreeing its not a healthy presence (???), I don't get it.
3. The immediate overreaction to Naga's presence in the tier. This is an odd one, the Pokemon wasn't even released for a day, yet people were claiming it was 100% mandatory to have AV TTar or SpD Heatran to beat it, I've played 70+ ladder games of USUM at 1800+ elo and have not used a single AV TTar or Heatran, this claim is completely false and it's just a case of builder-paranoia which puts everything into a vacuum and can make anything look like a legit threat if you look at it in the right way. I will even provide a list of all the teams I have used so far and can confirm that all of these teams work perfectly fine, despite not having any of the """"mandatory"""" Pokemon required to beat Naganadel.
4. Defensive answers. Albeit defensive answers are a little harder to find due to Naga's immunity to Toxic and ability to blow through all Unaware users, there's still viable options out there, SpD Tran is very solid at limiting Naga, as well as AV TTar (this mon isn't particularly good but it's found a niche in the tier so that's something, just because something encourages something out of the ordinary doesn't mean we should jump the gun!). There's also Chansey which gets off 2 tosses leaving it very weak and Naga doesn't even come out with a guaranteed kill vs it, so interpret that however you want. Ditto has also seen use on bulkier teams, it's by no means mandatory, but it's an extremely good Naga deterrent, and in general is VERY effective in this offense-heavy meta we currently have.
edit: I also forgot to mention Stakataka which is very self explanatory as it can live +2 Fire Blast and +2 DDrake and set up Trick Room in Nagas face or just OHKO it with Gyro, which is a huge deal against most of the teams you'll find Naga on.
5. It's one dimensional. Scarf, Specs, and Life Orb are mediocre compared to Dragonium Z, granted they can always pick off random kills with surprise factor despite being bad, this can be applied to any high tier OU Pokemon. On top of this, after the meta has been given time to actually develop a bit, Pokemon are accommodating for it more, things like Keldeo are running Icy Wind more, other things like Natures Madness Bulu is more common and is very nice for punishing Naga considering how freely it comes in (another good point about Naga, it keeps some extremely annoying Pokemon in check like Bulu too). Also, for those that have been playing the ladder lately, you'll be familiar with the ridiculous amount of Trick Room teams on the ladder which generally hinders Naga teams a ton too.
With all that said, I really do not think Naganadel is a broken force in the tier, and rather should be welcomed as a fine addition to the tier, nerfing Toxapex which made the SM meta stale and extremely centralised around it for the longest time, making for a fine buff to offense. I think its sweeping capabilities are weaker than Magearnas, mostly due to the lack of versatility in comparison. Magearna has the potential for 5 different Z-moves in its sets, and 3 forms of set up, and an incredibly useful defensive set too, and has the most powerful scouting punishing capabilities in the entire tier.
