You are correct about pulling on a horizontal cross member, but both the Jeeps that I built plates for, and a couple others that I helped install, were pulling on the main boxed frame rails that run for and aft, and also supported by the crossbar. I'm not an engineer, but it all looked good to me.
All brackets bolted thru the boxed frames have backup plates that are the life line of the engineering. If a person were to short cut it and not use the backup plates, would be a real good reason for the frames giving way.
If the base plate itself comes apart, I would have to say the welder was not a welder at all !