they can move ( slow and purposeful) and cleanse an objective with their heavy flamer and bolters and then cpature the objective.
They are an elite unit, they cannot capture objectives.
their major bonus in these games is their deep strike rule, allowing them to deep strike in, shoot and them assault the objective.
Not correct. They can re-roll deepstrike scatter dice and must always start from reserve and always enter by deepstriking. All the other rules for deepstriking apply. They cannot assault after deepstriking.
For your average game, a LoTD squad is going to be points heavy. They have some good rules going for them, but they also have a few negatives too. If anything, they are fun to use but just expect to be severely outnumbered on the game table model wise if you use them.
There deepstrike rule is a little different. You can re-roll the scatter dice if you want, and they always start in reserves and thus always deepstrike onto the table, other than that, all the rules for deepstriking apply.
As for the best way to use them, they cannot hold an objective, but can contest one. Deepstrike them were they can sit on an objective with their fearlessness, relentlessness and the invul. save to make them a real thorn to remove.
They can also be anti armor or anti HQ unit if equipped with a meltagun and a missile launcher, multimelta or a lascannon, (all of which can be fired on the move due to slow and perp!) and deepstriking behind an armored formation or HQ group can be nasty. Basically, I wouldn't hesitate to drop a unit right in the middle of my opponents lines and start blasting away. With a 3+invul. save, fearless and slow and perp, they can move, shoot, absorb damage and stay to the last model thus causing some serious distractions and confusion in your opponents lines. Sure, that is one expensive distraction, but that is sort of there purpose really.