None of these cheap machines have rack + pinion or ballscrews, that's simply not available at this budget and if it is you'll end up with a machine so small that it is probably worthless. But you probably already knew that. You could retrofit it onto an existing machine but by the time you're done it would cost more than the original and the improvement would be too small to notice. While we're at it let's throw in servos and focus compensation as well as a movable Z axis... But now the machine is priced out of hobbyist territory. An aluminum open frame machine like this is not aimed at industry and so should come with lesser expectations. It doesn't even compensate for thermal expansion of the frame.

The belts are usually quite good in quality, contrary to your assertion belt drives do not accumulate backlash (though they will have some it is more or less constant as long as you don't lose steps, which normally should not happen), have a Kevlar component in them to remove a lot of the stretch issues that you'd have with cheaper belts and either the gantry is moved with two steppers or there is a cross gantry shaft which operates a passive gear (without a motor) on the other side. Obviously this isn't perfect, the shaft is long enough that it will see some torsion so when moving fast one side will lag a bit and when you come to a sudden stop you'll see some overshoot. But even at very high speeds and long series of repetitions (100's) I've yet to see any backlash or 'slop' that is visible or measurable with the tools that I currently have at my disposal. This is funny because I totally expected to find a measurable positioning error but given a nice micrometer I find the positioning error when the machine comes to a stop after many 100's of meters of travel to be < 0.1 mm and positioning error from the origin to any point on the machine to be well within the acceptable.

The thing that you will notice is that because of the open construction of the frames that the machines aren't going to be square 'out of the box', and you'll spend quite a bit of time getting them to be just so.

I don't have access to an interferometer but if I can get my hands on one for bit by borrowing one somewhere I'll do some measurements on it but for now my simple tests suffice to show that the machine is quite usable and produces output that is dimensionally accurate to the point that it makes zero sense to farm out jobs to professional laser cutting services.

If I were cutting metal (which you won't be doing with a diode laser for obvious reasons) it would be a different matter, but even there in sheet cutting the tolerances on larger work are different than they are with for instance a mill or a lathe. You won't be making any press-fit shafts with a machine like this, nor will you be cutting gears with 1 mm teeth. For that kind of work it just isn't the right tool, and lasercutting isn't the right process. If you want that kind of precision in sheetmetal you would probably either use a mill (but then your workpieces will likely be small) or you'd etch your workpiece after a photographic process to create a mask.

When working in wood, cardboard or textile the precision that these cheap machines offer is ample.