Electrically the tracks with the connectors for power might have capacitors for radio interference reasons, they're unnecessary for and cause problems with the digital signals.
With regards to the wheel-track geometry, Märklin hasn't changed that much and most modern rolling stock compatible with the "Märklin system" (commonly referred to as "3L"/"AC") should work on M-track. Only the small "industrial radius" tracks might be a bit problematic in general and longer cars can hit turnout signals (mostly a problem with long passenger cars that aren't shortened to 1/100 or 1/93.5 scale). Some manufacturers have created "universal" wheelsets, those might be prone to derailments.
Digital trains need clean rails for proper operation, m-track is more vulnerable to rust and really needs proper rust removal, especially after not being used for a while.|
AFAIK it's not that rare to use m-track for storage yards and "non-visible" track rather than throwing it away.