The stand seems really solid. I don't understand the limit for the maximum diagonal.
Does the 38'' ultrawide monitor have a 100 x 100 mm VESA mount?
Perhaps with a heavy monitor the mounting system will be harder to remove once it's in place...

I personally attached a 42.5 inch monitor to the bracket. Basically no problem in terms of weight, and the mounting also worked without any problems. The reason for the diagonal limit is probably because the sides of a large monitor can hit the wall if you give it a knock. This means that it is no longer quite as stable from the side. However, this is not a problem if it is hanging in a place where nobody can normally reach it.

If the monitor has one of the two specified Vesa sizes, this should not be a problem.