I farmed Swaleys on my previous farm. In the right place they can't be beaten; the right place being the uplands on poor ground / moss / fells.
I'm not sure how they fare on good ground. Possibly their lambs would grow too big for the somewhat narrow exit channel. I do know that you have to take care buying draft hill ewes to run on better ground - you can get magnesium staggers as their systems simply aren't used to the lush lowland grasses. So if you want hill sheep on good ground get them as lambs so that their systems adapt.
I think hill sheep also do not like to be too confined or forced to be too close to other sheep (ie, they like a low stocking density.) Their genes tell them they want wide open spaces!
In my opinion the offspring of a Swaledale ewe put to a Blue-faced Leicester tup, known as a North Country Mule, is an unparallelled ewe. A North Country Mule can also be the offspring of a Blackface ewe put to a Blue-faced Leicester tup, the Blackie in question could be a Scottish type (short legs, rather dumpy) or a Hexhamshire type (a bigger, rangier animal.) In my experience, the Blackie mules tend to be less tame - but probably do put a bit more frame on the lambs, and certainly a better fleece.