There are no seals in the camera but access for water is pretty limited by tight tolerances. Water access points would be VF windows, sync plugs, beneath wind lever & SS dials.
I've read about them being usable in the rain without harm but wouldn't buy one if I knew it had been used that way.
If I'm out in inclement weather, mine(M3) is inside my jacket & out for moments at a time.
Unless you purchase a specfic built 'water resistant' camera, water intrusion from rain will probably be your biggest problem. My M6 and M7's routinely go from sub freezing temperature back into heated housing wiht no problems but for rain I have either zip lock bags or carry a Leica Large R bag that has an inner water resistant seal. These cameras are not designed for water immersion and if used in a rain storm, that's the same thing. Snow seems to ok if as Posted you carry the camera stored in your jacket and take it out for exposures.
I'm afraid that there is no universale statement that one can make about M's in 'really bad weather' because I don't know what you mean by 'really bad weather'.-Dick
Use an umbrella, deep hood and point the lens down if there is a lot of wind. Wipe off any excess when in sheltered area.
But if you take the camera out from central heating on a cold day unless you keep it warm it will have internal condensation when it get to dew point, unless you store it in a dry box indoor, with seals and fresh silica gel.
Internal condensation can compromise the lube or cause corrosion. The electronics should be conformally coated to protect to a degree, but the battery contacts wont be protected.
On a wet day the camera will go down to the water temperature, unless your hand keeps it warmer...