I think that Pask is extremely viable, and Tank Commanders in general are still quite good (at least at this early stage). AV14 is pretty good for your Warlord, he boosts his tank's effectiveness quite nicely (mainly the dakka ones rather than the blasty ones, but since they're all so much cheaper while the Battle Tank remained the same price I guess that's what your were gunning for anyway), and if you don't need the infantry orders that much (if you're running a super Blob Squad with Priest and Inquisitor, for example), you could do a lot worse than the Tank Commanders.

Have used wyverns against bezerkers and cultists.

Suffice to say I will be using them a lot. The reroll on hit and wounds mean even power armour can't make that many saves.
As with everything in the guard though, its all about the support

Tbh I overlooked the Wyverns at first...But yeah, looking at them again, they seem to fulfill the same role as my griffons used too, but arguably better (and cheaper), since they re-roll to hit like the griffon, yet also re-roll to wound and fire twice as much. Not AP4, but meh, they ignore cover and re-roll to wound so will probably kill more than a griffon anyway.

Just need to work out how to convert a cheapish Wyvern turret and swap out my griffon mortars and colussus cannons (thankfully magnetised them! So no loss there) on my numerous chimera chasis. Job done. Would probably run them in 2's, no more. Probably best in lower points games where I can't squeeze in higher priced artillery or many Russ's. Definitely good against backfield cultists, horde armies, if you don't mind list tailoring.

The loss is that you can't snipe out models with them and the AP is worse, as well as the obvious prohibitor of Line of Sight, but the Wyvern's guns are probably superior in simple damage output.

Ah yes. I think that is quite a major problem actually. I wont be able to just dump a squadron in cover and not have to worry about LOS, which may leave them exposed or waste turns not shooting at all if terrain is BLOB heavy. Will have to think about this as I think they are probably being overhyped.

Crazy question - why are you guys saying it can't shoot out of LoS? P34 of my basic rule book says all barrage weapons can fire indirectly. Is there an exception somewhere I'm missing?

The Wyvern isn't Barrage.

On page 62 of the Codex, the armory description of Stormshard Mortar says it is barrage, as does the reference chart at the back of my book, as does page 22 of the White Dwarf which discusses the Wyvern. Are the English language books printed in NA printed at the same place as for Europe and Australia/NZ?

Yup. British spellings any everything. And I just looked, the Stormshard Mortar is barrage in the physical book.

I just checked my copy and yeah it says barrage....I like it again lol.