Most often applied to eldar units (banshees, hawks, spears, lolords, etc.), but sometimes used when discussing chaos marines, dark angels, and tau.
While you may think the blurb's got some hidden meaning, such is not the case. It's really forumitis for 'the unit is shit, but I want it to work so badly, and you totally need to buy a couple boxes of it, too.'
No, banshees aren't specialists. They want to be, but they aren't.
If they were specialists, they wouldn't need shooting to soften the target up, farseers to hand out doom, or larger numbers than the enemy.
Specialists don't need support to down the preferred target!
Sternguard are shooting specialists. If you're forced into a short-ranged firefight against sternguard, you die. Case closed.
Hammernators and tyranid warriors are 'beat-your-face-in'-specialists. If you're taking a charge from either, you die. They don't need support, characters, or whatever - you just die. End of story.
Banshees are horrible, but want to be dedicated anti-heavy infantry combat specialists. If you're charged by banshees, you'll lose some guys, then make them lose a bunch of girls for their troubles.
Should the eldar have some avengers 'softening you up' with shooting, and a farseer around to put doom on you, you aren't likely to die, because a ton of weak, non-specialist units working in 'synergy' is still not effective, and the direct opposite of how specialists normally operate.
Again; specialists don't need support to down the preferred target!
Obviously, not all options are ever gonna be good, nor will all units on offer be useful to you. So obvious and universally understood is this, everybody who's not afflicted by forumitis openly admits it.
Swooping hawks, reapers, possessed, thousand sons, firewarriors, marine scouts, n00b bikers, phoenix lords - I could go on. The list is as endless as the depths of stupid attached to forumitis.
The big question isn't why all these models have such poor rules. All things considered, that's completely irrelevant to the discussion.
No, the big question is why so many people seem intent on making new players pick up really bad units, that are still costed as highly money-wise as the good stuff.
Pretending that Farsight's good is really cute. He's your toy, after all - pretend he's whatever you feel like.
Telling people with no clue that Farsight is good isn't cute, however, but terribly bad, horribly irresponsible, and shows massive weakness of character.
Disregarding Farsight's extreme level of suck, let's pretend that Farsight is an actual speciaist unit. Even then, new players shouldn't be recommend to pick him up, because they're not qualified to use him for the intended role. Why? Because new players are new, and when you're new, you don't have much experience.
Despite all their claims to the contrary, people know this. Yes, Irisado knows that banshees, nightspinners, and phoenix lords are awful, and should be left in the case, but he's not gonna tell you this, and is withholding critical information for a very important, very personal reason: he wants you to suffer like he did.
A fun game to play - on warseer especially - is pointing out that all the bad units people push and crusade for are indeed bad, and not specialists.
The first and only response you'll get is the second part of the blurb; the 'you don't know how to use it'-piece.
People love making excuses, but they also love attempting to justify things. They'll honestly and truthfully bring up the 'avengers+banshees+farseer/doom=success'-example. This is know as 'combined tactics. ' You know, that one thing you're trying to avoid by bringing in actual specialists.
'Specialist units' are doubly bad when there's something else available that does the same job, and more still. Compare striking scorpions to banshees.
Both are known bad units, yes, but there's an important lesson to be learned here.
Scorpions wear power armor. This gives you much greater protection than carapace, thus ensuring your little predators will be fighting long after the banshees are dead, and that they don't poof instantly to heavy flamers. More value for your points, in other words.
Scorpions have strength 4, and 4 attacks each on the charge. Puny banshees are strength 3, with 3 hits each. Sure, the girls are lugging around power weapons, but does that really matter when they both kill pretty much the same number of marines on the charge, and the scorpions won't explode to the return blows? The answer is: no, it doesn't matter one bit.
In addition to all this, scorpions can field a single dude with a strength-lots superchainsword, or a powerfist. This gives you a fighting chance against armor - which the banshees can barely touch. What do the girls get? A mask which grants them initiative 10, that's almost completely useless, since few actual units in the game even top the scorpions' initiative 5.
"But banshees kill terminators!" No, they don't. They die to terminators just like everything else that's not elite infantry.
While their chances of beating terminators are a tiny bit larger than the scorpions', why would you ever consider throwing either unit against them? Really, now.
Besides beating down tactical squads, scorpions demolish most light infantry extremely quickly. Employ banshees for that. Watch them get swamped and die in seconds to orks, guardsmen, and gants.
Scorpions 'win,' because what they bring to the table is useful against several unit types - light armor, light infantry, small squads of heavy infantry.
Yeah, the scorpions are still horribly overcosted, poorly equipped, and not optimal for anything, but they can do something - unlike the banshees.
Banshees are part of that very exclusive club called 'useless units,' while scorpions are merely 'bad.' Bad squads in a unit made up almost entirely of bad units? Take the least bad.
Maybe the point forum-regulars are trying to make is that units like banshees, spears, and firewarriors are highly specialized in the art of failing, and we're bad for not being interested in knowing how to fail like true masters.