Frame strength of small TT's

Basic rule of thumb that is not a guarantee but an idea - ultra lite and lite trailers will have a thinner/smaller everything steel. That is the biggest weight saving a builder can get. That does not mean they are bad but just that there is less steel. Those trailers will also usually have less cargo carrying capacity so you do not overload it.
RVIA rules have changed also to ensure a safety margin on tires and axles. It appears rhe engineering for rv's is sound but the execution sometimes leaves much to be desired. Thus the recalls over the years.