Admittedly it's a fairly advanced usage for analytics, but the limitation we've found is that you can only use one value as a unique identifier e.g. you use a user's account number as a unique id for a visit but then if you want to calculate the conversion rate in a funnel all visits by that user have the same id and so there's no way to calculate the conversion rates per visit. It means you have to choose your events and properties incredibly carefully and often it means you have to record the same events multiple times with different values just so that you can run the calculations in the right way. This would be fine except that mixpanel isn't exactly cheap per data point.

Beyond this the other major sticking point is that you can't update data points. You have to get them right first time otherwise you need to blow the whole database away and start again. Depending on your requirements this can be an absolute killer. We've now resorting to using a version id in the event names so that we can blow the old series away (via hiding) and then repopulate using fresh data, far from ideal.

It's really frustrating because overall I think mixpanel has a fantastic product. Not being able to update data points and only being able to use one property as a unique identifier is proving to be a real PITA though. People have been asking for these things for years as well and there just doesn't seem to be any willingness in the team to address these fairly fundamental issues. I'd absolutely love to keep using them but having seen bamboo.io it's starting to get more difficult to justify the loyalty.