They are quite ammo-sensitive. With the right load they were pretty reliable. The original load used a truncated cone 124gn jacketed bullet.
One of the claims to fame of the Luger pistol, was that the 9mm Luger Parabellum (9x19mm) cartridge was created for this pistol and the pistol was officially adopted by Germany in 1908 as the PO8, along with the cartridge.
The Swiss had already adopted the Luger pistol in 7,65mm Luger caliber in 1902. The 7,65mm Luger was basically a shorter version of the 7,63mm Mauser bottle neck pistol cartridge as used in the C96 "Broomhandle" Mauser pistol. The 9mm Luger is a necked-up version of the 7,65mm Luger.
The 9mm Luger Parabellum cartridge seems to be still gaining in popularity worldwide.