Cheap stuff is worth what you pay for it.
Surplus military or civilian ammunition could be perfectly fine for any use, but you need to remember that it's "surplus" for a reason.
Frequently it's solely a matter of being "out of date" according to that particular military's own guidelines. Ammunition that's just a little too old for some nation's military might be perfectly fine for casual, or even target use. Cartridges that are really old, however, will be more likely to suffer misfires and squibs, so you have to pay more attention to them when you shoot them. An unnoticed squib followed by a fully-powered shot will wreck your gun—and maybe you, too.
It is also possible to buy surplus ammunition that is inherently dangerous, having been rejected by some agency because it's over-pressure. Or, somehow, this particular load is otherwise unsuitable for your particular gun.
Also sold as "surplus": some unknown person's reloaded brass. Stay away from this stuff as from the plague!
"You pays your money, and you takes your chance," as the old carney once said.