The downside is that you can’t level it at the machine, you have to level at the surface it’s sitting on (table, cabinet, whatever). That’s kind of obvious, I know, but the follow-on to that is that it makes it harder to precisely square the machine, depending on how you go about squaring the machine.
In my opinion (which is sometimes…crap…
) the best way to square the machine is with a machinists level. Here is how I think it should be done: X rail not level . You can’t do that without being able to level the base.
I’ve changed my mind on how to get the tram on the Z axis right (fore and aft) - it requires shims under the vertical rails with the stock axis setup (use leaves from a feeler gauge)
So, long answer, that’s the only real downside, and depending on how you’re squaring the machine, it’s not a big deal. A full torsion box under the machine is really the best thing to do, and if you go that far, being able to level the torsion box is an easy thing to add when you do it. The rest is normal after that.