I have got a Mio 687 and in the MioMore desktop application (v7.3) you can add custom POI's.
If I add more than a couple of hundred POIs (total places, not different catagories!!) the mapping system freezes. It will calculate a route, but as soon as the vehicle moves, the whole thing freezes and needs a reset to clear. The freeze occurs when ever the Mio is moving and stops at which ever point it has reached i.e. If you are moving, it freezes at the orange 'Loading Maps' screen.
It is deffinately the installed POI's as if they are all removed, it all works fine, but like I say, it will cope with a couple of hundred, but no more. I rang Mio who said it is supposed to cope with 12,000 which does not include the pre-loaded !