The problem with welds is they can weaken the metal right next to them. A poor quality weld on a minimal sized frame or tongue members can easily reduce the strength significantly and make failure likely. When I was designing my ttt design a friend said to not let welds any where near the joints between the tongue and frame because they are some of the highest stress points on the frame. He also said all bolt holes need to be in the middle third of the vertical webs of the frame members unless the design takes them into account. None on the top or bottom faces unless the overall strength of the member was increased to account for them. If necessary, use a bracket to transfer the load between the tongue and frame. The problem with putting a weld or bolt hole through a top or bottom face of a C-channel, I-beam, U-channel or box beam is those operate in tension and compression so any change in strength will be a stress point, and may lead to failure there. On the other hand, the vertical webs just operate in moderate compression and tension in the middle third of their height. The top third can be thought of as part of the top face, and the bottom third as part of the bottom face, and thus are under high tension and compression. This doesn't mean you can knock large portions of that middle third out. it just means that putting a 1/4" diameter hole in the middle of a 2.5" high web may only drop the overall beam's strength 5% instead of 10%.
I ramble a bit, and I'm no metallurgist or engineer, just wise enough to seek out and listen to what they have to say.