We have an Arctic Fox 30U of 2010 vintage. It has performed well. However, the Four Seasons certification is overblown. In really cold weather, they will freeze up just like anything else (Canadian winter below minus 20 Celsius) though much depends on the specific model and floor plan. I have learned how to address this issue if you want more information.
As to frame strength, they are not bad. Some of their TT units, like ours, have a problem with the A frame in the front end. The front crossmember tends to bend if you use a high capacity load leveling hitch.
Overall, I am happy with our purchase. They are indeed structurally sound for the most part. Fit and finish inside are no better than industry-standard.
A lot of things can be retrofitted on an RV. Structure and frame cannot be retrofitted. In that area, Northwood excels. Would I buy another unit from Northwood? Without hesitation. In fact, I probably would not look elsewhere.