Relying on 6's is not reliable at all, even with re-rolls. I've seen plenty of matches where a squad of TankHunter broadsides, +MissileDrones is shooting an AV-13 vehcile.
Then out of the 22 or so hits rolling only a single 6, including the re-rolls from tank-hunter. These things happen ofcourse, as they are forever part of a d6-based game.
My point here being, we don't shoot spammed Str7 at Imp.Knights, hoping to kill them. sure, they can damage it, but picking between 2 glances, or removing models somewhere, I'd pick the latter everytime.
The same for WraithKnights, When relying on 6's, and you have the option of more reliable wounds elsewhere, I'd go for reliable wounds every time.
Toughness 8 is a jerk, mostly because we Tau don't have any weaponry on the S9/ap3 class. We're literally forced on spamming wounds from MissilePods, RailRifles and Seekermissles
Missilepods are easily avaible, but will need staggering amounts of hits, this requires markerlights. so becomes expensive pointswise.
RailRifles are very limited in terms of availability... quite literally only on Broadsides, and still only wounding on that 4+
Seekermissiles are easily avaible, and in general, extremely usefull versus Eldar (particulary Jetbikes). But considering these also only wound a W.Knight on a 4+, you'll need atleast 12 hits to deal the 6 wounds required, and thats not even counting FNP.
We pay points for every missile, some basic math shows we'll need over 100 points in seekermissiles to kill that Wraitknight, I'd hardly call that efficient.
There's always Skyrays however... These guys are cheap enough for what they offer. AV-13 with Markerlights and Seekermissiles. the Skyfire and secondary weapon are just extra bonusses at this point. Remember that Scatterlasers are S6, so cannot glance the AV13!
I've recently been inspired by 2 Skyray-heavy lists, and am currently planning on taking 3+ skyrays by default against any Eldar opponent.