The fact that it is different than real sleep isn't really contested much. There have been comments that they are supposed to be great for mid-shift guards at the very least because of trance. The hard part is how to adjudicate it.
I would use the Sleep spell as a baseline for sleeping characters and make them unconscious. Consider their passive perception as -5 (for disadvantage) when it comes to noticing what's going on and waking up. This means a very low wisdom character has a chance to sleep through a fight outside their tent. (Likely a DC 5 as a very simple task).
Trance, on the other hand, flat out states that it is different from sleep in the description. It is a meditation state, not a true sleep, and only has to be maintained for four hours. I would say that this also gives disadvantage as their focus is shifted internally, but this would be easily negated by the generally high perception that elves have. They would hear combat and normal noise regardless, but wouldn't likely hear an assassin.
All this said, the duration is the real prize. Elves don't need a full night's sleep to get their rest, so they can be alert for half of a long rest. During this time frame, they are not suffering from any penalties to their perception. Two elves can cover an entire watch with no worries at all about being asleep. This combined with their natural alertness makes it hard to ambush a group with at least 2 elves in it. (And fairly difficult to ambush them even if it's only 1.)