The Review -
Dark Reapers are the poster boy of what not to be in 5th edition. Immobile, medicore durability through squad size and an ability they pay through the nose for which has minimal impact within the current ruleset (AP3). Everything you could want for a 35 point infantry model! These serious negatives have really seen the downfall of Dark Reapers despite being in an edition when the most common enemy is what they are designed to obliterate - Marines. The simple fact is, cover is too easy to get to half the effectiveness of Dark Reapers and Dark Reapers are too expensive to suffer such a blow to really be viable options.
Dark Reapers are also hampered by their relative durability issues and immobility. At T3/3+ they are decent survivable but due to small squad sizes (max of five), they cannot take serious punishment. Having heavy weapons also means they aren't moving and shooting - something which is very important for denying the opponent cover. This means Dark Reapers are vulnerable to being nullified by terrain and opponent moves and also being in danger of engaged quickly by the opponent.
At unit sizes of three to five you really want to max them out - they are expensive but without a great amount of durability you need those extra squad members to keep the unit functioning and to protect the most important upgrade the unit has - the Exarch. The Exarch turns the unit from terrible to bad - so it's certainly a step up. With a variety of weapon options (most notably the Eldar Missile Launcher and the Tempest Launcher) and abilities (Fast Shot for increased rate of fire or Crack Shot for ignoring cover), the Exarch can actually hand out quite a bit of damage against a variety of foes. The EML gives the unit the actual capacity to damage tanks (and Fast Shot doubles this) above AV11 and can still help with infantry clearing thanks to small blasts (though with AP4, dumping AP3 wounds due to wound allocation comes in). The Tempest Launcher with Crack Shot makes mince meat of any infantry not a Terminator but doesn't improve the anti-tank ability of the unit at all. THe issue here is the Exarch is often an 80+ point model and is still only T3/W1/3+.
Potential Uses -
Dark Reapers provide nice anti-infantry fire - forcing Marines to always make saves on a 4+ from cover if not out-right dying is nice if not exactly earth shattering. The big issue is just its so expensive to do this and there are such better options within the Heavy Support section.
That being said, if you want anti-infantry firepower, Dark Reapers can provide it. The Exarch in particular with the Tempest Launcher can crack shot can tear apart anything bar Terminators and fast shot provides the unit with some utility against larger infantry squads. It's still an expensive and inefficient proposition but it will damage any opponent who clumps infantry together or gets caught in the open (i.e. after transport destruction).
Potential Changes -
The most basic changes here need to make Dark Reapers not just an anti-meq unit - it's too specific a role to see use in balanced lists. Yes Aspect Warriors are supposed to provide a specific role but a specific role against specific armies? Pass. With this in mind something like an altnerative firing mode would allow Dark Reapers to provide anti-infantry support from long-range without being drastically inefficient against horde infantry. Something like a S5/AP3 Heavy 2 & S4/AP4 Heavy 3/4 would allow them to drop Marines in and out of cover (remembering cover is likely going to a 5+) and Guardsmen/Orks/etc in larger quantities.
Adding a potential Rending option to one or both firing modes allows for some vehicle suppression but nothing really reliable. Potentially not needed based on what the Exarch is capable of. A decrease in points is obviously going to be needed as well with the potential addition of Slow & Purposeful to provide some mobility to the unit. Exarch powers counting for the whole squad will see the unit as a whole get much better and could potentially override the need for altnerate firing modes (i.e. crack shot for MEQ, fast shot for GEQ).
If Magun Ra also allows Dark Reapers to see time as Troops they could make quite a decent backfield scoring element - they wouldn't be a super fantastic choice as they aren't cheap or numerous but could take them out of the Heavy Support section where they are out-classed by heavier options.
In the end, Dark Reapers cost too much and do too little. The Exarch can throw out the hurt against multiple targets and if you catch a unit in the open they can cause some heavy damage against even Marines but even then, you're paying through the nose for relatively situationally effective firepower. They aren't really capable of dropping hordes quickly without decent scatters from the Exarch's blast weapons and do diddly squat against vehicles unless the Exarch brings a double shooting EML. They are only T3/3+ with a max of five bodies and will ultimately die to inconsequtional ranged firepower (i.e. couple Psybacks, Heavy Bolters, Multi-lasers, Scatter Lasers, etc.).
To make them more useful providing more options against multiple infantry types, not just Marines, is necessary whilst also decreasing their cost and raising their mobility. This allows them to not just be static gun emplacements and impact the game on a wider basis. You don't want them super cheap obviously - but cheaper than a Rhino at least.