DE aren't exactly hated but they are a bit rare due to three reasons:
a) Lots of crap models - they aren't all terrible but plenty of them are ugly as sin
b) A very very very fragile army. Although they hit hard and fast, just 1 or 2 mistakes can easily cost you the game. DE have absolutely zero staying power vs shooting, and only a select couple units can survive in CC too.
c) Simply put, there is 1 excellent unit (Wyches), 3 good-to-middling units (Warriors, Ravagers, and the uber-killy HQs), and the rest are mediocre-to-crap (reavers are bloody expensive for what they do, Talos are too slow to fit in with the rest of the army, Warp Beasts are alright on the charge but tiny squad size, scourges are just utter crap, and so on)
this means that all DE armies fall into 2 categories:
a) Raider Rush - lots of warriors and wyches and ravagers. Doesn't work too well unless you have heavily terrained board or are fighting someone with next-to-no long-range firepower.
a) WWP Rush. You shuffle your warrior units/WWP carriers forward for a couple turns, then unleash the wyches/ravagers/warp beasts/some (myself included) bring jetbikes too.
Not a lot of variation in lists = a generally "boring" army t'be honest. Fun to play every so often but every game ends up looking very similar after a while. This is the biggest problem with the army - pretty much every DE list consists of:
1 Killy Lord or 1 cheap WWP carrier or 1 of each.
2-5 Warriors with Dark Lances (in raider if Raider Rush, on foot if WWPing, or a combo of both) and 2-3 Wyches in raiders if Cabal. If it's a Cult list, it's 2-5 Wyches and 2-3 warriors.
Usually Warp Beasts - 1 in Cabal, 1-4 in Cult.
Reaver Jetbikes/Ravagers/Talos to taste to fill up the rest of the points. Generally Ravagers in "shooty" lists, Talos as "Fire Magnets", Reavers as either 3-man squads with 2 blasters for mobile anti-tank or slightly larger squads used as CC support for Wyches/Lord or taking out min-maxed las/plas marines/dev squads/etc.
You get fringe cases with Mandrakes, Hellions, Grotesques (as an alternative retinue for the Archon), and Scourges, but those are pretty rare.
You can win a lot of games truth be told - the combination of lots of BS4 lance weapons, the incredible deadliness of Wyches (if you can keep them in CC at all times) and the HQs (DE Archons+Incubi or Archite+Wyches are the deadliest CC squads in the game, depending on target), and the speed at which the army can hit you - especially when using the WWP assault - makes them an army capable of completely wiping opponents off the board be they Marines, Crons, Nids, Tau, or anything else (exception being "I have 90% of my points of unkillable skimmers+troops hiding in them and I'm going to hide them out of LoS until turn6 when I'm going to 24" move them to contest objectives - and the other 10% are jetbikes that are also going to hide. I won't win, but you won't either" Eldar wanklists).
But when it goes wrong, DE tend to get absolutely pasted - some games you'll see you warriors melting under pie plates before getting to shoot and your wyches completely bouncing off of the weakest units. Poor positioning will often lead to complete failure.
They make a great tournament army under the (IMO, rather shitty) "20-0" system simply because they're an army that tends to massacre or be massacred - you don't get many draws or minor wins with or against DE. Under a (IMO more fair) "3pts for a win, 1 for a draw, 0 for a loss" scoring system they don't excel quite so well since more solid, dependable, but less-likely-to-massacre-their-opponents armies can score more points overall with minor wins. Also tournaments often have lots of MEqs and MEqs is what DE are best at killing - they have very little chance at the points denial Eldar armies and Hordes can be tricky (although are still beatable) but MEqs get absolutely slaughtered.
Also, even the good units are going to get utterly shafted by 5th ed rules. the sniper squads are going to get their lances killed very quickly, the area terrain no longer blocking LoS makes their units even more likely to wither under fire, the change in LD for losing combat means the DE will run away even more than they currently do, and the lack of "killing zones" in CC means wyches are going to get completely torn apart by return attacks they wouldn't normally suffer, and so on.