Carbon Bars: I have used the Race Face Next Carbon low rise bar for four years on three different bikes...still going strong, no complaints.
Carbon Seat Post: That is one product I have seen (and been part of) multiple failures of...I had an older Easton EC 90 the frayed and split on a ride, and I've seen them sheared clean on friend's bikes as well.
I'm a Thompson seatpost and stem man for life. It's the gold standard no matter what the criteria is-there are some products that are just the best-THompson Seatpost and Stems are one of them.
I own (and I am looking to sell) a USE Alien Suspension Seatpost-27.5. You can check the mtbr reviews on this product, it is very highly regarded. It does add a ton of 'comfort' and seated control with little fuss to a hard tail....if that's what you're looking for.
Bigger tires run tubeless are one of the keys to comfortable all mountain bliss. Josh is right about that 100%.
I also use Ergon grips (small)---Some folks love 'em, some hate them, they're not for everyone, but for me, they added a ton of comfort, reduction in arm, wrist, and neck pain. I won't ride anything else for grips.
Liam