using 'black' pipe for building projects???

Well, it depends.

Price: varies with the market and the supplier. Usually pricier around here for the strength, but not always.

Ease of use: Here square tube and angle tend to win, as the cuts don't need fishmouthing to weld a joint, but I have several carts at work that are made of pipe, with creative bending to minimize the welded joints.

Resist temptation to build a cart using threaded fittings. Seems like a great idea at the time, but banging around tends to loosen them. Weld type fittings, on the other hand, can really simplify joint design, for frames and carts.

Personnaly, I tend to build carts out of the scrap pile, so I use whatever is handy when I need to built one. I pay a lot more attention to the wheels than to the steel. (If it will be outside, biggest wheels I can. Weight rating for the wheels is also key. Swivel Vs. fixed wheels. etc. etc.)

Pipe cuts and welds fine. It's not great stuff to machine, but if you hold it rigidly, it will work OK. The trickiest part is fitting pipe together at odd angles. I often make a handle from pipe welded to two flanges, like in this picture:

Black pipe is fine for projects but most people use what ever is in the scrap bin, with pipe it is likely to be galvanzied pipe. And the brings all the issues with zinc fume fever as well as cruddy joints if you don't get the coating properly ground or burned off.

Pipe joints are more work than using square tube or angle.

If you are by> I think building a cart for the OA rig is a priority.

Recently made a hand truck out of 1.5" black pipe. Strong as a tank. Heavy as a tank as well.