Are all 12-gram CO2 cartridges the same?

Functionally, yes. All 12-gram cartridges hold the same 12 grams of liquid CO2 at the same vapor pressure, regardless of brand.

Real differences are quality control on the seal, weld, and neck dimensions. Some compact pistols are picky about cartridge length and neck taper. Threaded versions exist for specific platforms. Buying in bulk from established brands (Crosman, Umarex, Leland) keeps the per-cartridge cost reasonable.