Here's the short version on those 2:
Nightwing - I either haven't figured out how to use it right, it suffers from painted model syndrome (it's a nice bright sea blue and green with a pink seahorse on the nose, picked it up second hand) or it's one of the few skimmers in a mostly infantry list. In the several games I've used it at 1500 and 1850, it's only ever accounted for a single Killa Kan. That said, in the weird meta I play in, the only time it encountered AV13+ was against the sisters, I brought it on from my board edge, was 4" short to engage the exorcist, and then on the following turn it hit the flimsy AV10 with 3 or 4 Krak Missiles and it exploded. I haven't written it off yet, but I just don't see the heavy armour threats that justify taking one. Mathematically, it should be superior to a Pulse Hornet against Rhinos, the catch is you have to get it into 24" to use the Shuriken Cannons, which is generally all kinds of a bad idea.
You may need to proxy your own and give it a try since my results are not generalizeable.
Phoenix - Somewhat points inefficient. I've only thrown it down for the 1850 tournament, and I think larger games is where it will remain. Game one it came in from reserves two turns from the end. It's first turn, it did a number on a squad of devastators clumped up in cover from 48". The second turn I had to waste it's firepower to clear a single scout marine off an objective, and it was all for nought because my corsair squad holding an objective decided that it was time to run. In Game two I got the phoenix on turn two. On it's first turn it cracked open a sisters Rhino with the Pulse laser, next turn the Nightfire Missiles scored some obscene amount of hits on the dominion or sisters squad bunched up in the crater of their former ride (I think every blast caused 4 or 5 hits) and then the following turn it whittled a squad of seraphim down to one. It took a lot of fire that game. Game three against footslogging orks, it came in on turn 3, but by that point most of the board was covered in greenskins. Every deployment option was bad, bringing me within 24" of shootas. It did manage to cut a 30 boy mob down to 14. I was ready for the reprisal from the shootas, but a single lucky rokkit brought it down.
While the results might not sound impressive, given that the Phoenix was on the board for maybe 6 of 14 game turns, I think it did pretty well. It's why I mentioned in my initial AAR that I really need to deploy it on the table rather than holding it in reserves. I suspect it will work even better when I have my bullet sponge and two warp hunters to take some of the heat off it.
The Tau player's lineup seems like a really bad matchup for what you were throwing down. A pair of broadsides with SMS would have made a world of a difference over the stealthsuits methinks. That or going more Fireknife Crisis heavy...Ah just thinking of how I would've done it makes me want a Tau army again...