The issue with PS2 DVD playback was that the format changed and you needed to update firmware to ensure largest coverage of format differences. This was before the consoles were hooked to vendor networks, so you had to buy the update with a DVD remote. This led to the impression that the PS2 wasn't a reliable DVD player.
These days, format changes are handled by firmware updates over PSN. This ensures that the vast majority, if not all, Blu-Ray discs will play on a properly functioning PS3 (by which I mean, the laser isn't dirty or some-such). If you're having an issue, it is very likely either hardware issues (like a dirty lens), firmware issues (like disconnected from PSN for too long), or disc issues (like a bad pressing).
Hope this helps.