I think it was a bigger concern with earlier generations of pistols that were not set up to handle +P loads, but even then it appears that it was generally safe in reasonable doses. The evolution of the BHP into the Mk II and Mk III versions, for instance, was apparently driven at least in part by the fact that the SAS were cracking slides shooting lots of 9x19 ammo loaded for SMGs through their WW2-vintage Inglis HPs. The guns were failing after prolonged and heavy flat range and shoot house work, not after one or two rounds fired off the first magazine loaded with SMG ammo.
That said, like an earlier poster noted, my understanding on the matter is that "SMG ammo" can describe a whole range of pressure levels, etc., depending on the nation in question and probably the time frame when the ammo was loaded as well.
Because of this, I would avoid using such ammunition except in an emergency unless you know hard numbers on the loading to avoid any potentially unpleasant discoveries. Probably not an issue, but it might be, and having your front teeth knocked out by a slide or something would be a rather unpleasant way to learn about quality control in ammo or handgun manufacture.
