Just did a whole house that has had what you described, though it wasn't
White it was Green.... Must admit most came off pretty easy with a wall
paper striper. But in places that it didn't want to come off it just didn't,
(Luckily it wasn't massive areas) just really tricky ones, like the toilet
being the smallest room in the house I managed to turn the entire room into
a Turkish sauna. The paint went to some gooey consistency and got
everywhere. It my experience those roller spiky things didn't really do much
apart from leave marks on the walls. Best thing I found was to score gently
with a sharp Stanley careful not to attack the plaster.
Good luck its not a nice job if it doesn't come of easily
J