They can still die during the final assault, I think.
True story. Loyalty only seems to be half of a character's chances of surviving the suicide mission, while the other half is from your character upgrades or simply who you have doing what. I had total loyalty from everyone on my Renegade playthrough, but lost Thane because I used Miranda during the swarm part - Jack died because I didn't upgrade enough of the Normandy and I didn't trust Morinth.
Apparently even if you do EVERYthing (max loyalty, ship upgrades, assign the proper characters to the proper tasks), there's still a possibility you might lose some of your teammates during the last big battle (you and your task force are fighting the final boss while the rest of the crew is holding the line).
I was lucky in that I'm kind of a completionist so I would check every planet in every system to see if there were any anomalies or missions, before continuing on to the main loyalty missions. That way, I had a ton of resources so I could easily afford the upgrades, and my character's levels had risen enough that I was getting 100% bonuses for paragon/renegade actions.
So by the time I got to the arguments that arose between certain crew members (which happen totally without warning lol), I was able to defuse the situations with no problems.
The more I think about it, the more I come to believe Mass Effect 2 is the best game I've played on the 360. Amazing, amazing experience. Sucks so much knowing I'll probably have to wait two years for part 3.
