Not saying it's broken for sure but I'd bet a pex suspect would be a lot less one-sided than the melm one.
Pex can be annoying yes, but I don't think it really crosses that threshold. It has a similar issue to Melmetal defensively, it has to spread itself out to check specific threats and isn't always the end-all be-all of defensive options when considering regenerator mons. We have specific tools such as trappers like Fini and Heatran, things that naturally threaten it and are very common like Tapu Koko, Zapdos, Lele, Washtom, Landorus, Garchomp, Nidoking, etc., and wallbreakers that threaten to pulverize teams like Dragapult, Blacephalon, Gengar, Crawdaunt, and Kartana. I think given the wide array of tools we have there is really no reason to complain about Toxapex aside from miniscule interactions, it needs prepared for.
On the subject of boiling down concerns to nothing more than skill issue, specifically on boots, there is some truth to the fact that it needs to be adjusted to. However, I don't necessarily think boots are all the problem as regenerator could be, even if the two go hand-in-hand as I have previously mentioned. Boots have given more viability to mons that would have otherwise been pretty stunted this generation like Volcarona and Dragonite, while providing certain defensive options like Tornadus and Zapdos with more of a buffer to deal with offensive threats that run the tier. Slowbro and Slowking hold a special place seeing as we've almost had issues with them before, though again that was mostly Teleport + Future Sight and they have since toned down as the meta matured. Offensive options such as Victini, Weavile, Moltres-G, Tapu Koko, and Zeraora have also benefitted from boots, and none of them really present any major issues because of it.
I understand and respect some of the reasoning behind wanting to look into Toxapex, but there has been no real indication that it is broken or unhealthy in a meta where we have plenty of answers to it. We've even seen it decrease in usage over previous months and eventually trends such as Gastrodon and Rotom-W sprang up and became more popular which further contributed to a decline in usage. I think the meta has done nothing less than prove that it can adapt to Toxapex naturally with the available tools it has in a healthy way that isn't overbearing or restrictive.
Given that reasoning, I don't think Toxapex would be a difficult case as much as it would be open and shut because the majority of the community recognizes that it poses no real problem. Same for boots really, it's just the way that things are in the generation 8. Things are no better or worse than previous generations due to either of these, it's just different.
(P.S.)
That said though, it isn't impossible to see a Toxapex or Boots test before the popularity of this generation dies down. However, I think we all know how it would go. It's mainly just a peace of mind thing.