No it’s not, it might be cheaper but if you have access to a good printer you can’t possibly say that building a lower out of LEGO and cardboard is easier.

Even completing an 80% lower with a dremel tool is harder than loading a file and hitting print.

In my experience, any 3d printed item needs a lot of work to make it functional. It isn't as easy as hitting print and walking away.

Making one with a router and a cutting board is tedious, but I've done it and the result was functional. Making one with Lego and epoxy I have not done, but I know someone that has. It isn't like it takes days.

Depends on the printer with a high end printer you can print a lower and go with no work needed other than to drill the holes for the FCG and magazine and bolt catches.

With a production or industrial prototyping printer you don’t need to do anything than drop your parts in.

Heck it wouldn’t surprise me if in a few years with multi material printing you’ll be able to print a fully functional lower even with the FCG I’ve already seen metal parinting demos that can print both rigid and spring metal materials in the same print.