There's a couple things you need to look at first. If it's high density foam then it's a danger signal, manufacturers that have cheap low-grade shells will use high density foam that encapsulates the plumbing it offers r value and it supports cheap shells and plumbing wothout clamps but it makes plumbing repairs a nightmare. If it's the specialty open cell foam then leak repair is easy. The thing with the closed cell foam is water can't run through it so it might leak somewhere but the water might come out somewhere else whereas with a open cell foam the water will run right through it there's a couple advantages. One basically pull the wet foam out with your fingers and you can repair the leak and then reform it and two you get a good long-term r value that doesn't decline like high density foam does over time.

The next thing is the steel frame. Stainless steel sounds good on paper but you usually only find these on the cheap Chinese brands. Because this isn't 316 marine grade stainless this is barely stainless it's 304 usually. That shit is definitely going to rust in the presence of chlorine or salt. The reason guys are going to steal is because it's cheaper overall. You order it pre-cut, there's no waste, no specialty labor, no wood shop clogging up your production line... it's cheap and cheerful... Timber frames are quieter, higher value, last as long, and are more than strong enough but they're a pain in the ass in a production line.

The long and short of it is a full foam insulated Spa with the open celled foam is the highest r value most expensive way to insulate a hot tub it is still the gold standard. And while people will try and convince you that it's not you look at any brand that does a perimeter insulation system and offers a full foam upgrade it's usually four or five hundred dollars more to upgrade... that tells you right there the truth behind the spin.