Increasing in size on lowlands isn't a problem unless you let it get out of hand. A slight increase is beneficial though if you are marketing them for meat rather than showing. Don't let them get too big as they really aren't designed for it.
Start off by buying off farm, avoiding the hefty sale prices, unless you want showing stock or have mega bucks to spend.
Don't buy anything that has had to have it's feet trimmed. The majority have good feet, but like all sheep there are some that don't. Avoid breeders who have neglected to cull bad feet.
They aren't invincible, they can still succumb to the usual sheep ailments, including fly strike if you let them get dirty (although they don't get dirty very often, at least mine don't
). Actually I've seen strike on clean BWM sheep on shoulders etc and around horns (but these weren't my sheep).
As for Fronhaul's statement about lambing late - around here (SE Wales) they will lamb early Feb without persuasion. It obviously depends where you live.
Most are very gentle in nature, especially the boys. They do like to climb, and, as Fronhaul says, un-tie things (shoe laces esp.). I've not had any go wandering off though. Mind tend to stay together like a family. Apparently if you put them in with white sheep they stick together too and don't mingle with the Whites. lol.
The only con I can think of is having to stop mid-drenching/shearing/injecting/whatever-job-it-is to talk to people who have spotted your sheep and stopped to talk to you, saying they are "so lovely" and want to know all about them!!!