you have to take into account that for stealth to be successful, the level design needs to be built around that mechanic. i think one of the reasons it worked in c1 was because of the level of openness the game allowed for. looking back at deus ex, there's always at least 3 different ways to get from one section to the next, and so even in what seemed to be very linear sections, there was always an opportunity to take a different approach if you felt like it.
c2 is so linear that it barely allows for anything else than just one type of gameplay, namely gunplay. stealth really feels like it is the unloved twin in the family. i haven't gotten to the end of c2, but in many of the sections i went through i was experiencing huge problems with taking alternate routes around patrols and soldiers, and many times i was just straight up forced to try and stealth past them, often with them noticing me just as i went by. ironically, my best c2 stealth encounter was at the helicopter boss in the lab, where while the soldiers knew i was there still couldn't find me. this approach was also a bit forced due to the sheer amount of damage rocket launchers would do.