First off hello there. Let me start by saying that us Tyranids actually don't even have a single flyer. FLYING Monstrous creatures are our version and they have a few subtle differences. These include a shorter required distance to move (with the option of running,) being able to fire in ALL directions unlike the directional shots of others, and we have to take grounded tests everytime we get HIT (not wounded.. so yes a markerlight or lasgun can drop you and inflict the automatic S9 hit, forcing you to ground. Also as per the last FAQ this is our only way to get skyfire in our codex, seeing as we got excluded from being able to use fortifications guns. The Harpy is our only FMC by default loadout as well, but our Tyrant can become one with a wings upgrade.
If the other flyers overwhelmed you then don't worry, ours are almost harmless. The Harpy is terrible at lower than average MC toughness, armour and toughness (making vector strike pointless.) It also has blast weapons, (so dogfighting is impossible,) and needs to be babysat by synapse. Although I have proxied them a few times just to try, they have never done anything productive.
The Tyrants on the other hand are a little more interesting. They are going to cost around 225-250 though so you better hope you picked the Brainleech Devourers to have a chance at shooting the other flyers. At least 12 Twin-linked S6 shots have a chance of glancing most flyers out of the sky. Pair this with biomancy and it CAN be scary. The only thing is all those other flyers have skyfire too and 3+ isn`t going to help against anything shooting at it.
I give the Flyrant a 9.5 out of 10 and the Harpy a 3. Hopefully this helped a little.