In my experience, the stability of a trailer is far more dependent on the distance of the tow ball coupler to the axle, and having adequate tongue weight, than anything else.
Once that distance gets too short, things are difficult to stabilize. The 4x4 versions of those cheap trailers are the worst, and the "boat trailer" versions are the best. (ever notice how boat trailer people can tow constantly faster than almost anyone else? long ball to axle ratio is the key.)
I agree, those 45 MPH ratings are a lawyers CYA statement. U-Haul does the same on many of their towing rigs.
One other thing that those trailers often have that can be a fatal weakness is the tongue construction. When they are a simple inverted "U", if you ever have the load shift to negative tongue weight, it can spreed and "collapse" upwards folding the tongue into an angle until the tail of the trailer drags. (ask me how I know this...)
Mine was bought about 25 years ago (Same basic design) and the only part of the original trailer still in use is the axle and the springs. After the tongue collapsed on me, I rebuilt the whole thing and extended it as well.
In its current configuration I've towed it well past 75 many times, especially with my old full sized truck, as with that truck you could not tell you were towing anything at all with that trailer. BTW that trailer has a small tent trailer box on it and is when fully loaded with camp gear is about 1000-1200 lbs.
I will also say I don't trust many of the 8x4 tires made today at all... YMMV