It is remote work, so think a lot about whether you are ready to work from home 100% of the time, if you don't already. Set up a separate space (preferably a different room as your 'office', if you can) to work in, so you can maintain a healthy mental work-life balance.
Holacracy isn't for everybody. If you want to be told what to do / directly managed, instead of holding yourself accountable for your own projects, then you may not like it. However, if you do like it, it will ruin you in that you will probably never be satisfied by other companies if you ever leave (I assume; I haven't left).
The workload for the small team ebbs and flows, so you won't have a completely full plate all of the time. If you can't find productive ways to make yourself useful otherwise (learning, helping out with presales, or helping with marketing efforts for example), you may feel frustrated. This expectation is set at the outset during interviews, so you shouldn't be surprised by it really.
The ESOP doesn't kick in for a while, so you will have a gap in any employer-based contributions to a retirement plan/account. This isn't a big deal if you can just contribute to a personal IRA or something like that in the meanwhile.