M+S (Mud and Snow) tyres - disadvantages?

Depends - if you're looking at all season tyres with M+S rating, they should be OK but aren't that good on snow.

Proper winter tyres with M+S rating with be more noisy and will wear faster when it's warm due to their softer compound.

Hooch (Hooch ) gurgled happily, sounding much like they were saying:

Worse grip & faster wear in normal tarmac conditions, IME.

Isn't faster wear usually related to better grip? I thought that softer compounds gave better grip, but resulted in faster wear rate, and vice versa.

Given the same tread pattern, yes. M&S tyres have a very aggressive tread pattern however. They don't really excel at anything, and generally have poor dry grip.

I would only consider using them in a rural situation where some limited off road driving was necessary.

Chris

Yeah...

I ended up arse-ending two cars on wet days. Absolutely sod all grip in the wet on tarmac.

Only if half of the potential surface area isn't missing because of the wide grooving.

If everything else is the same. Proper snow tyres are silica based rather than rubber, so they're stickier when cold, not so good when hot & wear faster. There's lots of different things you can trade off as well as wear vs grip.

Noise will be worse. Other than that, careful choice of a reasonable compromise shouldn't lose you too much.

The compound will be harder than pure tarmac compounds, but not excessively so (unless it's some crazy offroad tyre). This also has the slight advantage of good wear mileage, but barely so you'd notice. Unless you go for a particularly hard compound though, you shouldn't have any problem with lack of grip on wet tarmac. As M+S tyres are generally narrower than current fashions, they might even be better.

Not really much use in London then, given the usual absence of any real winter weather? Apparently only 3% of motorists in the UK use winter tyres.