danceman, I agree with all that you said.
Some nuance however:
DE are nice indeed, for DE players, obviously not for DE opponents! I know only goblins to be nice for the opponent's point of view!
DE ARE balanced, with the meaning that they behave well in all phases, but NOT with the meaning that they are in the average of WH armies.
You need not to assess your point over, nobody disputes that DE are a top tier army now. Here the consensus seems that DE are ranked #3 among all WH armies, everyone is convinced here that DE is a powerful tool.
Yes, superior maneuvrability is forgiving (but you have to know how to use it).
Yes, hatred makes the first round of melee more reliable.
Yes, the dagger makes our magic the most reliable (although there is no more immunity to miscast item).
Yes, assassins are a nice surprise and don't take a character slot (however they are expensive, vulnerable and cannot take any magic weapon save venom sword...).
Yes, they are strong builds, such as shade death star or ASF BG.
However, NO, nobody can possibly make up a list with simultaneously all of the avaliable advantages: manoeuvre, melee, magic, shooting, monsters, ASF BG, lots of assassins, whatever...
So, even if our opponents call cheese for all of the advantages that we COULD bring in, there cannot ever be the full choice of cheese available, and the elven vulnerability (fragile and costly troops) remains, which is why DE player pretend that what they DID bring is not that cheesy!
Also, as already mentioned, a powerful tool needs a skillfull artist to make good use of its full potential.
People would assume that racing against against a Ferrarri is hopeless, however, let me be the Ferrarri's driver and then your Mercedes is enough to win! Only if you drove a Lada would I bet I would win the race.
Same goes for DE: Now, they are powerful and balanced (in the meaning that they are strong in all phases) but they are simultaneously vulnerable (except a couple of units: hydra, ASF BG, dragonlord).
A good general (especially the one trained with 6th edition DE) has a precision powertool in his hands.
An average general has a powertool in his hands, which makes his life easier than with a less powerful tool, but it's not enough by itself to win all battles.
It takes both a good horse and a good rider to win races (or an incredible horse with a competent rider or an incredible rider with a competent horse).
It takes either a bad horse or a bad rider to loose races.
So usually people blame the horse.