I'm not sure why you experienced that but on my end I've received great support from the Mixpanel team. In fact I remember one case in particular where we were aliasing our users wrong and a team member there got on a call at 1am to resolve it.

Mixpanel is one of the few pieces of software I've been excited to pay for. Hate to see them get a bad rap.

It seems like there may be certain start-ups being targeted that aren't a perfect fit for the pricing structure. If you are B2B SaaS the price is well worth it.