The downside to a Series LLC is the legislature created a caveat to the firewall that is associated with a sub-series. The firewall only exists if the records maintained for a particular sub-series account for the assets associated with that sub-series separately from the other assets of the LLC or any other sub-series. It should not be difficult to comply with this caveat, but you may have to prove compliance every time a sub-series is sued. Proving compliance will likely require a hearing along with the disclosure of financials (which will feel very intrusive especially when handing over to an adversary).