Obviously not a great one since your using unsupported rails
I bought my rails a good few years ago now before supported rail became affordable to the DIY scene. It is too easy to put what is a practical use down because of listening to misinformed information. From a reality point of view i have not seen anything like what Jonathan is insinuating about using unsupported rail. It is also putting people off even trying to build an affordable machine that may well have been suitable for them. A few people have spoken to me asking for a realistic picture of using round rail because they had access to it. They have been worried that they may get shot down for doing something bad.

now if you were going to span a 20mm rail over 1.2m then you are asking for trouble. It is all about using the right thing for the right job so comparing the same size of rails against each other is pointless.

As for my job i do system engineering not mechanical but my rail was spec'd by an mech engineer which is why it works.