I've heard numbers like 5 microns or 10 microns. In my business we own and operate "real" CNC machines and in many of our parts are often machined with tolerances of 0.0001 (2.5 microns). This is a standard tolerance for a press or interference fit or for a bearing bore, etc. I know how difficult it is to maintain such a tolerance... it requires a well built and well maintained machine with ball screws and precision encoders and precision drives, and you have to account for thermal growth of the machine.....
I come from a similar place. Mechanical engineer that owned/operated 5 axis CNC shop and designed high precision motion systems. It is very easy to get 'paper performance' of a motion system that does not account for all the real world details. I also have Quads like @Corporate666 has and they would likely struggle with these numbers even though the motion system is vastly more sophisticated and mechanically sound. Before I got my own P&P machine - I was totally naive of the process. It is remarkably unforgiving. What seems like a small problem at first, becomes a huge delay where the machine is doing nothing and 3 people are fiddling with it for an hour. That means the reliability has to be amazing. You will not have useful reliability without amazing feeders, accuracy, repeat-ability, rigidity, and more.
Just as a reference, my Quad is rated for 3600cph in which it's actual placement is significantly lower since I do not perfectly optimized the feeder layout. My best burst speed on hand placing is about 100cph or so after taking all elements into account (I know you can place the same 0805 passive much faster). After all is said and done - the P&P is only about 3-5x faster overall for small runs of boards assuming the programming is all ready to go and most of the feeders are ready. So, the placement speed is 26x faster but the process is at best about 5x faster. That will vary for each user and different design, but it does illustrate the importance of reliability. If anything on the machine is fiddly, or not accurate, or not reliable in some way - it will very quickly eat the time benefit of having a P&P machine. It took me a while to get my system reliable enough to be useful and it is a commercial machine. It only takes a tiny problem to be a big problem and that is the challenge of low-cost desktop machines.
the nature of the P&P process means it is almost exclusively a business tool - not sure why a hobbyist would need 100's or 1000's of PCB's made if they were not for sale. Being a business too, it needs to be highly reliable since you will be making promises to customers based on the expected performance of the P&P system. I am skilled in the art of mechanical design and motion systems, very high accuracy CNC, electrical engineering, and of course the practical every day reality of running an in-house P&P line for small volume. I have not yet seen anything in the desktop class that would make much sense for anything other than the most basic projects. These days, a 'basic' project can use very tiny parts - 0201, 0402, BGA, .4mm QFN, etc. Personally, I would not bother with anything that cannot guarantee success with those parts with very high reliability. It would be lower cost to spend $20k on a machine that gets it done perfect every time than a $4k machine that sort of gets it done. I did not have $20k, so I went with a used eBay machine and slowly added feeders - all in cost $10k although it is a fluke that cannot easily be repeated.
None of this is intended to discourage anyone. The goal is to put the practical reality in perspective. P&P machines do NOT assemble boards. They merely help in the process. Even an amazing machine needs a lot of work and skill to make it practically useful. A fiddly machine needs even more work and skill and the end result may in fact be worse than manual assembly. The entire process requires perfection at every stage.