I used to have a Woodrat and now I have a Router Boss. I use them to try and replace some of the dozens of jigs that are required for chairmaking. They both delivered against that brief, but the Router Boss does it better. In fact the Router Boss does it much, much better.
A key part of the problem was that the original Woodrat's moving carriage simply wasn't engineered to a high enough level. It wasn't too bad for my applications because I only need a small amount of travel, say 350mm or less, and it was possible to shim the carriage to be accurate over that span, but if it was accurate at one end of the full travel it would be out at the other end, which must have made many users tear their hair out with frustration! In fairness I believe a later version has a beefed up design which should certainly help. I'm surprised though that you're having so many problems with M&T joints, are you sure the components were accurately machined before jointing, that your router cutters are really sharp, and that you're not trying to plunge too deep with each pass?
By the way, the Router Boss isn't without its problems, for example the laser guide is woefully inaccurate and it also suffers from the same "stiction", tooling, and capacity issues that limit the usefulness of the Woodrat. However, it's inherent rigidity, and the superbly engineered tilting table that makes quick work of the complex compound angles you find in chair construction, means I find it's worth persevering with. It's not cheap, but given that it genuinely allows repeat machining to better than a tenth of a degree and better than 0.1mm, makes it pretty good value for money.
