Riiiight, welcome to your own twilight zone.
Well, given that I presumably have much more experience with piracy than you, I think I'm more qualified to judge the reliability of pirated games. Let me break it down.
The pirating community is called "the scene." There are numerous groups who compete with one another in order to crack games first. Prestige and all. Now, in order to ensure some level of quality in these releases, there are rules and standards that have to be followed. All games must be retail source and live up to their label. A cloned image of a disc doesn't need to be cracked, but it must be playable through drive emulation or other means. A cracked game must be playable without any emulation or disc, but must also retain the exact same functionality of the uncracked game. If a group fails to meet these requirements, they are officially "nuked" until they release a fix or another group releases a proper.
Needless to say, I only get official scene releases. Now, if you are some newb who downloads games off BT or Kazaa, you may not be downloading these scene releases. As such, you may encounter faulty cracks. But that's due to your own inexperience more than anything else.
So yeah, properly cracked games are always as stable as the retail releases and sometimes more stable if the crack removes the obnoxious copy protection.