The studs are hard. They will penetrate a fully frozen layer of ice hiding under a dusting of snow. That's the whole point. They give you a secure ride in cold weather, and even if there is no ice almost everywhere on your route, it's the 5-meter concealed stretch of ice that can take you down.
Granite and cobblestones
The lesser problem is noise, but even if you don't mind the continuous clickety-clack of studs hitting the tarmac, you are reducing the area of rubber in contact with the tarmac. Most of the time you'll be fine. The contact will be good enough. If you do venture over granite, or over cobblestones, the studs will not help grip at all because they cannot penetrate the ground. Then you're just reducing the area of the rubber that contacts the ground.
Do they brake acceptably?
They do brake acceptably, but if you brake hard you will be prone to losing a couple of studs on every ride. You'll lose one once in a while anyway; just not that quickly. Also, if you look closely at the studs, you'll find that ice, snow, and tarmac leave them with no sign of abrasion, but after braking hard you will see sign of the tarmac acting like sandpaper on the studs.
Sand over the tarmac
Anyone who ever attempted to turn, even midly, riding on tarmac covered with a layer of sand has learned that rubber tires provide zero friction over sand. Here again the metal studs help by gripping the tarmac underneath the sand. Snow tends to attract dirt and sand, and when it melts in the middle of winter it tends to leave behind, if not a layer of sand, at least patches of dirt.
The last point is yet another argument to leave winter tires on for the season, especially since it's tedious to switch the tires on the same wheels. Mounting summer and winter tires on different wheels saves time only if the rear wheels are exactly identical, since otherwise you'll have to factor in the time it takes to retune the rear derailleur.