It's hard to say. On the one hand, there are plenty of things that stop it, as it's a slow Calm Minder with no defense against status or Taunt unless you run RestTalk, but that sacrifices coverage.
On the other hand, we've never had a Calm Minder, or even a bulky Water with this much sheer bulk before. It's solid enough to be a definite competitor for a teamslot, and some teams don't even have a means of dealing with it. What hyper offense team runs Toxic?
I think that, once people start to break out of the mindset of 'Mega Slowbro is a Calm Minder' and start to try different sets, it'll become more popular. This is the Avalugg with better typing that we all wanted. The Mega Aggron with Recover. The OU Suicune.
Its got enough bulk and special attacking power to shine in OU. A bulky set of Slack Off, Scald, Ice Beam, and Fire Blast, or an Offensive Trick Room set is just as viable, if not even more than the Calm Minder. You can even hold off on Mega Evolving so that Slowbro keeps Regenerator during the earlygame scouting and pivoting phase.
If your opponent doesn't have a Mega Manectric, Mega Sceptile, Thundurus, Magnezone, Toxic user, ect. they might not even be able to break it.
Note that Reuniclus was almost banned last gen, but that was mainly due to Magic Guard. Shell Armor is a whole new kettle of fish.
