Hey Tom,
Not every universal motor comes with the danger of burning and set your workshop in fire. If this was the case, such motors would not be ubiquitous but forbidden. A universal motor can start to burn if you overload it so that it is slowed down ((see also difference router/spindle) because then it will get too hot and malfunction (it’s also written in Makita manual). That is surely not to expect to happen with a shop vac. But it will be VERY CERTAINLY the case if you use a hand trim router like the Makita RT0701C or RT0702C that is designed for short-time, hand-held usage and you mount it into a CNC and run it for hours, especially with a bit diameter of more than 1/2" (like those big 1½" surfacing bits). That were the conditions when the Makita started to burn which was reported many times in this forum.
By the way, the 43 mm “Euro” mount milling motors for CNCs that are common in Europe and that I consider to be the best milling motor solution for hobbyists and semiprofessionals because they need no VFD are universal motors with carbon brushes too. But I NEVER heard of one of them starting to burn. That is because they are not a Makita hand trim router that is designed for short, hand-held usage, but dedicated milling motors for use in a CNC for 24/7.