There are many semi-displacement hulls more than capable of handling very rough seas. The limitation many face is range, but just look at yachts like Westport and Hatteras and Heesen. Our Riva could cross but only has range of 200-250 nm.
Elling has good sea worthiness but not significantly more than some other boats in it's range, just more promoted. Sort of like Nordhavn, with sea worthiness, but no more capable of ocean crossing than several other brands in it's size range.
The self righting is half impressive and half a gimmick. Ideal controlled conditions. Doesn't mean if you flip in the middle of the ocean, you're going to self right and just continue on your merry way but does mean the boat has excellent stability and ability to handle waves.
Many semi-displacement boats handle rough seas as well or better than comparable full displacement. A couple of days ago when the Miami show let out, all the Viking and Hatteras boats were moving north. They were all running offshore in 6' of seas at 25-30 knots sort of in a SF caravan. There was a total group of about 8 or 9 boats ranging from the 45' Hatteras to the 90'. Oh, must add too that the 45' Hatteras and the 41' Cabo both handle rough seas incredibly well from all the reports I get.
We intend to one day cross the Atlantic and the Pacific and it will be in a much larger semi-displacement or better called a semi-planing boat capable of 25 knots, cruises at 20 knots, but the crossing will have to be at 10-15 knots due to range.
Most of the "trawler" owners I see on the East Coast do the majority of their cruising in the ICW. Many consider 3' or 4' waves to be bad. That has far more to do with their tolerance for rough seas than it does with the capabilities of the boats. Staying inside also is based on their lack of speed making outside runs more time consuming rather than less. Then as to comfort, they're unable to adjust speed to achieve more comfortable matches to the wave heights and periods. They may struggle with aft or stern waves while a semi-displacement would just speed up slightly to eliminate the issue.
All these labels of displacement and semi-displacement and trawler and fast trawler and planing and semi-planing really say nothing about sea worthiness.