They're middile teir at the moment. They got a nice boost from 6th edition. While most armies got a nice boost, Tyranids got a more favorable boost in regards to Flyrants becoming
FMCs (and thus absolutely deadly with dual devourers) and the sheer amount of psykers they can field. An average army, between Tyrant, Tervigons, Zoanthropes and Broodlords has ~5 psykers.
They suffer in the same places they did in 5th though - a lot of vulnerability to S8+/AP3- weapons, and monstrous creatures are still too expensive for what they can do outside of the Tervigon and Swarmlord.
Oh very much so. While they got a nice power boost, you still need to maneuvre forces to take advantage of Fearless, a few bad rolls on the Biomancy table leaves you without much needed support, and without proper cover, your synapse and monstrous creatures will evaporate.
Fearless from Synapse is now a straight up buff, rather than the dubious ability it was in 5th. Multiassaults with supporting monstrous creatures can work now due to the monstrous creatures not fainting from seeing a few gaunts die. Large broods are now quite viable. As long as they're in synapse, if a brood of 30 Hormagaunts makes it into combat, you have to kill every single one of them, which isn't easy if the synapse creature is a Zoanthrope throwing Endurance on them.
There's units in the book you'll notice to avoid right away - Rippers, Sky Slashers, Pyrovores, etc. You'll know whem when you see them.
My main advice is
experiment. While The Internet likes to tell you it's only worth taking Tervigons, Trygons, Termagants, Genestealers, Hive Guard, Tyranid Primes and Hive Tyrants, this is based on the 5th edition tournament meta. We're in a new edition, and your local group probably plays differently to the tournament crowd. Use those recommendations as a basis, but try 'middling' units from the book.
In particular - my group isn't big on ranged anti tank, because no one runs pure mechanised armies. So I get away with using Carnifexes and Tyranid Warriors, which don't instantly evaporate. I get huge use out of Zoanthropes, particularly now with Biomancy, while I keep getting told they're too unreliable. There's a lot of units in the book that jusst aren't good enough to make it in the tournament scene, but there's a good chance you'll have a use in your local group.
Except Pyrovores. You won't find a use for them. No one has.