With older CD based systems, I have been pleasantly surprised at how many work even today. Out of the 4 Model 2 Sega CD's I've owned, one had a bad laser. Considering the age of the systems, I'm not going to complain about that. One did have a bad motherboard though (Wasn't the fuse; stopped working after it had been dropped by the prior owner). Same for the PS1's I've run across; most work fine. The 2 Saturns and 4 Dreamcast's I've had all worked perfectly.
On the other hand, of the 8 Phat PS2 systems, all have had disk read problems. It was mostly bad lasers (1 had a dying motor), but three could play PS2 games, but not PS1 or CD's. Highly annoying. 3 original Xbox systems, 1 has a bad laser, one is starting to have issues with the tray sticking. And we shall not speak of the my first 360...
Of course, this is just my experience. I got many of these systems a decade or more after they were produced, so I have no way of knowing how they were treated. But it seems that the quality of the components has gone down over time, expecially in the case of the PS2 lasers.
-Rob