I love the Stegabow.
I think that the bow's range (and its impact) brings something to the Seraphon shooting phase that is largely missing from our warscrolls. While the Bastiladon's gem offers up similar shooting power, the two monsters, themselves, fill dramatically different roles.
It's unique in that way.
Back in 2016, I had my fun with them: putting one on the board for some early target-softening/positioning usually put my opponents in quite a mental tangle.
Often, my Stegadon was handled like a support piece and would survive into the mid-/late-game. And, that makes sense. It's kind of an odd profile.
On the one hand, it was a model that often wasn't worth engaging early/at a distance (especially when it was hanging *behind* approaching infantry units).
The bow seems fairly non-threatening in the bog picture, and the Stegadon just needs to be softened at most to be handleable in combat. It'd be an overcommitment to bring it down, right? There are better targets, right? It's not like it's necessarily hard to wound the Stegadon...
On the other hand, when that bow hits, it freaking hurts. And it keeps getting closer! So, ok, maybe throw a few early spells or shots its way (and away from, say, the Slann).
Most opted for the second choice: chaffing my original Stegabow, usually sending something out of position to do it.
But then I'd try to summon another Stegabow, and then all hell would break loose because they've now shifted their support role into a drive-the-masses-before-us role!
One was on the board the whole time, plucking away, not a huge threat... and now there's a buddy, suddenly changing the board state in a big way.
Two Bastiladons couldn't do quite the same thing imo. Just different roles.