It doesn't have to be solely a performance condition.
I don't think it would be hard to prevail on the facts arguing that a requirement for an IPO (or similar liquidity event) is wholly out of the employee's control and that a substantial risk of that not happening occurs up until the moment that it actually happens. IPOs fall apart/are withdrawn and mergers fail frequently enough that a substantial risk argument could probably be sustained.
I don't think it would be hard to prevail on the facts arguing that a requirement for an IPO (or similar liquidity event) is wholly out of the employee's control and that a substantial risk of that not happening occurs up until the moment that it actually happens. IPOs fall apart/are withdrawn and mergers fail frequently enough that a substantial risk argument could probably be sustained.