It's too vulnerable. You've got a maneuverable army with wargs and maruders which want be exploiting that and engaging on their terms.
However it's also very low on numbers and a single model, no matter how big or powerful, won't be able to protect it. Only one(might be two not sure) model needs to be tied up from the crew and it won't be able to fire. Which with archery and how you're likely to be outnumbered won't be too difficult.
Even if siege machines are left to their own devices it's very unlikely they'll make their points back. I'd love to include dwarf ballista's in my dwarf army yet theory hammer says they'll never make their points back
. This is all speculation though, I've never had the joy to play with siege engines .
