Looking at dump trailers... any mfg to avoid? Prices are crazy,.. GN vs. Tag?
I bought my first dump trailer in '96, 5 trailers later I still don't have it right

I have learned a lot but am still learning, so share your findings.
I recommend a gooseneck if you tow heavy and often. Bumper pull for lighter loads and seldom use. Research is your friend as is look in person. Most of mine I bought sight unseen as dealers are few around here.
Go with name brand. Mom and pop have poor paint facilities and tend to cheap out. Some big companies are guilty as well. There is so much to look at and consider. Diamond C is top notch but way over priced. I don't like their tail gate. Even on the tall sides they only use two hinges on the barn doors, one s-cam lock and the spreader chain it too high on the gate.
Note the picture above, I have one just like it. PJ 7x16x3 22k. The bed overhang is too much. When you dump it can hit the ground if the ground is soft, or the truck is on a higher plain. Just the weight shift rocks the bed on the suspension and unloads the pin, that brings in closer to the ground.
Bed length is important, especially on a bumper pull. The longer the better, it helps distribute the weight to the hitch. 14' 14k gvw is good for most uses. Bed floor and sides. 10 gauge minimum for the floor and 12 on the sides. If you haul logs with pig ears go 7 gauge all over, but it adds weight real fast.
Telescopic lift or scissors. Stay away from the single or dual cylinders. They are all slow, the telescopic will lift anything but it is the slowest. Make sure the scissors is mounted at least 16" ahead of center or it will struggle.
Tarp kit, load ramps, spare. Hydraulic jacks and brakes add big $$ but are nice. 8k axles or more are spendy, as is torsion. Spring axles have a 5-6 year warranty, torsion 10-11 years.