Hey I'm a long time 40K player, I've been playing DE for about 6 months or less.
This is what I've learned so far:
Dark Lances are ok against most armor..against AV14 you will struggle.Unless you are one of those high rollers.
I have found wyches in a venom with haywire grenades are the way to kill armor.
Throw one on the way in and then glance it to death..it does the trick.
Blaster are your friend in KW squads..more lance shots the better.
Splinter cannons are better and cheaper than DL's..more shots at range for those monstrous creatures and demons.
The crucible isn't reliable against psykers other than warlocks and other low leadership..it can work though and people don't expect it.
Sometimes gimmicks can work.
As for Tau..nightshields will help,but remember the riptards have 72 inch range and blacksun filters.
Also god help you if some one brings a broadside squad with heavy rail rifles,your vehicles will go up in smoke very fast..night fight wont help you(black sun filter,tooled up commander with no cover save BS).
More than likely you wont see railheads,if you do rethink your tactics and pray.
I had this happen to me at a friendly game(damn near was tabled),but not at a tourney.
You will need allies in any kind of big tourney.
I plan on running DE/Eldar at NOVA this year.,I'm taking out cousins with me.
They are a spoiler army and it's psych out job just pulling them out.
I think your right about demons and Eldar..Tau are where we will struggle.
Hope I helped,hell I ramble a lot!
I need my pain,it helps me focus on setting your world a flame.