Whats the hardest part of Tube Chassis builds?

[BR3W CITY]

Say a guy has some welders, and some DOM tubing or box...how hard is it really to build a tube chassis (or backhalf even for discussion)?

To me it would appear a lot like doing a cage, but a lot more time intensive and waaay more sensitive to plumb/level/square. You don't see a ton on DIY tube chassis for certain obvious reasons, like not killing yourself...but I know its something a competent hotrodder and welder should be able to do .