The legends don't actually roam, they're all confined to one specific area. That said, they are still pain in the asses to actually catch because of awkward mechanics or just straight up ass arenas. Landorous and Thundurus aren't too hard to catch, but Tornadus and Enamorous just suck ass, ESPECIALLY Enamorous holy **** is she terrible to catch.
As for space time distortion mons, just catch the first stage evos. Don't bother with the rest, grind satchels to buy the evo items, it's much faster and far less RNG dependent