Downside: you'll need to build all visualization by yourself. Fortunately that's easy with tools such as Looker, Mode, Metabase etc
Upside: you can do with your data whatever you want - built any reports, join with other datasets etc. You not limited by reports MixPanel team build.
In reality, Jitsu and MixPanel could co-exist. Jitsu support s MixPanel as a destination (e.g. you send data to Jitsu ; Jitsu sends it to MixPanel and data warehouse).
This is not really clear from the website. Mixpanel is mentioned in https://jitsu.com/sources but NOT in https://jitsu.com/destinations. Also the docs seem very clear about that.
We are currently looking at Segment, Jitsu and others. While we generally liked Jitsu, this was kind of a big deal for us and made us lean towards Segment.
However, no final decision has been made yet. ;-)