As the DM here, I have a few thoughts:
1) The spell, IME, needs to be changed in a fundamental way. The aforementioned 17th level druid is nothing compared to the 20th level druid, who could easily have a few thousand of these guardians.
2) One possible change would be to add a Material Component to this spell--namely, a living plant that's at least 15' tall. So a druid could theoretically convert part of their grove into guardians, essentially awakening them into shambling mounds for a period of time. But doing thousands would be a pretty epic event, and would essentially disrupt an entire forest for a few months. It would also make the spell pretty sucky in the desert, or for indoors combat, etc.
3) The closest analogous power seems to be an evil cleric's ability to control undead. With enough Command undead attempts and Animate Dead spells, he can control up to 5 times his HD in undead minions. Would it be appropriate to put a similar limit on druids (meaning a 17th level druid could control 85 HD worth of shamblers, or 7 of them)?
4) The last, and perhaps most restrictive limit I'm considering, is that only one casting of Shambler can be in effect for a druid at any given time.
Ultimately, it's not a spell I see being abused a lot. So I'm not that worried. But I'd love to hear your suggestions!
Nareau