I think it's reasonable to be balanced. I'd say instead of treating it like an interview, treat it like a performance review. Identify what the company does well and what could be improved and/or what would have kept you from looking.
I recall two cases where employees were honest with the company about why they left and in both cases management took their comments extremely serious. In one case there were issues with management which were addressed and in the other it was more process and work balance focused which also resulted in a different approach from senior management.
I think in Calgary at least, the market for good people is very competitive and companies hate losing someone they spent time on. The good companies want to know if they could have prevented you from leaving.
__________________
|