I think the Beltex lambing issues are more pure Beltex than Beltex cross.
However...
I've lambed a lot of NC mules (Swaledale x BFL) to Beltex and Texel and found the mothers very roomy, so births needing assistance were easy to do. Only problems were overfed single-bearers in good years - after which we fed single-bearing mules no cake at all unless it was a truly dreadful year (like this last one has been.) Sticking point for lambs was shoulders.
Then moved here, where the ewes are Hexhamshire Blackface x BFL mules and Texel types; births needing assistance are harder to do in these less roomy ewes, and the increased %age of Texel genes makes the lambs more muscley so harder to get out. Sticking point is less shoulders than rumps, although the shoulders are a considerably tighter fit than in the Swaley mules. Here we don't generally try to pull a lamb out unless we have both its front legs; in the Swaley mules you could get them out if you had one front leg and the nose.
I've no experience of Suffolk mules so can't compare their roominess to a Swaley mule - but the Suffolk is a meat breed, unlike the Swale, so you can assume the lambs will be considerably more muscley than the Swaley mule x Beltex.
A local purebred Beltex breeder, after experimenting with a loaned tup lamb last year, bought a Shetland tup to use on his Beltex hoggs this time. He said he'd get easily-lambed perfectly acceptable fat lambs that would come out running and get stuck into the milk bar whether the mother knew what to do or not. No stress for the mother, live lambs to rear and sell, and the ewes would know their job when they're tupped to the Beltex next time. Previously he was having some difficult lambings, lambs giving up on suckling before the young mothers had settled to let them at it - a lot of work and heartache, not a good crop of lambs, and some of the hoggs didn't rear a lamb so still didn't know what to do next time.