With the rise of mech that is a staple of 5th edition, they may have taken a backward step on the battlefield, being pushed out of that all important Heavy Support Slot for more popular options, including Fire Prisms and Warwalkers.
The problem isn't so much that Dark Reapers cannot deal with mechanised forces, but rather it's also matter of their effectiveness in 5th edition when they actually do manage to target a unit on foot. With all those units with 2+ saves, and FNP, with cover saves all around these days, it's no wonder that Dark Reapers have found a more suitable place on the shelves.
What I'd like to see one day, in our next codex update is an option to give Dark Reapers an anti-mech role - perhaps able to switch back and forth between anti-Meq and anti-Mech alike. This way, they have suddenly become versatile, they have suddenly become useful in 5th edition. We've seen how their survivabilty was upped since our last codex, and I for one welcomed the increase of our 4+ save to a more comfortable 3+ save. These days, your Reapers will sit in cover, hopefully pinging away through fire lanes or pockets where LoS is granted, but the real pain is that, despite their range, they can be easily countered with the right kind of positioning of enemy vehicles - and str 5 guns are of course wasted on vehicles. Give them an anti-mech option and at once they have overcome this obstacle, and are viable in 5th edition.
These days, they're simply too static, both on paper, and on the battlefield. There is really only 1 single thing they can do - kill marines - and they do that quite well. Deny the Reapers that one target, and you've effectively put them out of the game. I argue that generally speaking, as an Aspect, their role is simple Long Range Support - but this simply needs to be expanded to bring them back into play. I want to see Long Range units that can deal with armour, is that too much to ask GW? :)
Of course I could be completely wrong in my assertions, after all - just because they haven't worked for me does not mean this is the case for other Eldar players. For example, Fritz's Vyperology has seen me use Vypers in my recent games, and if internet wisdom is anything to go by, they are entirely useless in 5th Edition. Fritz has effectively smashed this claim into a million pieces and proven that a disregarded unit can not only find a place in 5th Edition, but work very well.
Any thoughts on Dark Reapers? Do they work for you?