That's only for DKoK Siege Regiments using the Siege of Vraks book for their army list (who also don't have access to the Wyvern, nor any psykers within the army list). Otherwise they're Heavy Support for normal IG armies and you can only take up to 3 per FoC slot.
If you get into combat with the guns, you're fighting WS3 S3 T3 I3 Ld7 5+sv guardsmen, certainly no harder to kill than the Wyverns. The guns (and their T7) are entirely ignored in CC. Wyverns can't be swept either.2) a S7 model (and through artillery rules helping your sqaud too) you can put in cover, has a different kind of survivability than a vehicle with an av10 facing, which I take down in combat constantly.
Sure, you can get some S3 lasguns, but they're not going to scare much.3) Overwatch - Wyrvn = 0, Thudd gun = at least one crewmen per battery shooting, for another 56 points put some more crewman in there for a lot more...
Again, WS3 S3 T3 I3 Ld7 5+sv guardsmen aren't usually going to survive anything that'll kill a tank unless you're talking about a single SM sergeant with a powerfist all by his lonesome or something.4) close combat, the ability to survive it due to the ability of crewman to fight around the models.
With a Wyvern you don't need the psyker or IC and you can deploy them without having to worry about concentrating your units around the IC bubbles. The Wyvern gets to ignore cover and twin link all of its shots, and not worry about Ld, without any external support. Also, they can move if needs be.only thing else is like the wyrvn and a psyker, keeping an IC around with stubborn/fearleass/zealot is where it gets to be a rule tough cookie.