Don't get me wrong, it is a nice frame, I'd trade for it in an instant and will probably never own one as nice.
To my eye and based on that single photo there is a long stretch of tubing mid chassis without much to give it torsional rigidity. I discount the trans cross member as a structural unit based on appearance. I also think these welds will be highly stressed if the chassis is worked hard:
No question it is stronger than stock and probably perfectly fine for a moderate build. Is it stiff enough to live up to the potential of the suspension? Or the horsepower an enthusiast may want twisting against some sticky tires?
A well braced structural cross member near back of cab.... would give something to hang the exhaust on at very least and make the frame appear more rigid. An automotive engineer might run the numbers of those box sections and tell me to take a hike.