A level 1 barbarian is going to likely have AC 13 or better, 13-14 HP, and take half damage from melee combat. A level 1 fighter/paladin tank is going to have AC 18 (chain plus shield), and 12-13 HP. Under the UA, a level 1 druid tank is likely to have AC 13 and 9-10 HP.
In comparison to the barbarian, that druid will get hit at least as often and be able to last about a third as long in melee. The fighter/paladin will get hit far less often and will have slightly more HP. A tank cleric could also have the high AC and self heals, with the same HP as the druid.
Under this change, the druid is not a viable tank any longer. Not remotely - you might just as well put a Warlock or Rogue on the front line for all the difference it will make.
Obviously, moon druid tanking is currently OP at low levels, specifically levels 2-4. Beyond that, they are more used as off-tanks, not primary tanks. So we are have taken a sledgehammer to them to fix a 3 level problem. There has to be a better solution.