sounds about as frustrating as my own experience against OldCrons last night!

a few thoughts:
- did the BTS FW start on OW? They could have protected much of the army and certainly themselves as nothing coming out of the portals could have weathered their AP. Instead they didn't seem to do anything of note
- Stealth suits. Probably better to wait until turn 2 to teleport them when there would be stuff to use their AP attacks on. As I'm sure you're aware AP disrupt vs Necrons is very good, especially vs newcrons.
- Stealth suits. Once they had been teleported surely you had to use them? even with a BM they would still be going in against an unprepared pylon on even terms. Get in CC and that's the worst it could be. Even when you had prepped the pylon surely it was worth risking the retain as even failing you could BM and break the pylon rather than leaving the stealth suits exposed?
- Recon. How were they deployed? Standard tau tactics round these parts, and even more so vs necrons, are to screen more expensive units with recon. In your game the necrons seemed able to move up and assault targets relatively unhindered.
- Intermingling - any army that intermingles 3 formations is going to have problems, but we all make these kind of mistakes!
Any list where you quickly lose 4 formations (5 including the railheads that were isolated to prep the pylon...), are not completely familiar with what your opponents army can do (and we've all been there!!), and seemingly make little attempt to stop him doing what he wants is going to have a hard time...
I'm also not sure about your terrain comment, I doubt using less terrain would have been any advantage to the Tau.... and looking at your pics, I think we normally use at least that much here in the UK... do you guys play TLoS or area terrain that all blocks LoS?