I currently have these barrel adjusters on my bike:
https://www.amazon.com/Jagwire-Rocket...ef=pd_sim_sg_2
one end of it sits in housing cup on frame, on the downtube

One of the adjuster is bent, no idea how did it happen, so it is time to replace it.
Is there any advantage using inline barrel adjuster, like this one?
https://www.amazon.com/Jagwire-Inline...?ie=UTF8&psc=1

Not enough difference to lose any sleep over but most of the time the 'Rocket II" in your top link only require one hand to adjust although I've seen some that can be a little loose and still need two hands. Oh, and some find them too bulky looking if you're bothered by that type of thing. For most riders these would rarely be used once the cables are all set up anyways so take your pick.

it's possible the advantage may be that it's less likely to get bent?

6 of one, a half dozen of the other. Tall people may find the inline ones a bit more conveniently located. OTOH, built on ones make for a cleaner look (until they bend).

the inline ones need four ferrules for a given span of cable where the rocket ones need two.

It's hard to find barrel adjusters that don't drift when you set them. I have a set of Rocket II barrel adjusters on the road bike, the front der constantly loosens and I find myself tightening it every few rides. Is there a good way to tighten up the barrel adjuster mechanism so it doesn't drift while keeping it adjustable?

lock-tite or similar on the threads?

hmm good point.. you could argue that the inline ones require 4 more since the you'll still need one at the frame cable stop too

I thought I did argue that. :/