Early stage or tech company acquisitions are primarily for people (and tech). So having an acceleration clause due to M&A for key team members might be a down side. If the person is leaving a well paying job, then he/she might want it.
For terminations, it doesn’t make sense to provide a golden parachute. If the person is leaving a well paying stable job and joining a startup, then he/she should really believe in you and know the risk of joining the startup. If you really want to compromise, you can put the acceleration clause for termination **without cause** post acquisition. So that on acquisition, the clause it carried forward as part of the negotiation, however know that it may be renegotiated. Note, California is at-will employment state. He/She can also leave you without notice. Finally, initial employees are time sensitive. If you feel this person is super sensitive to your business right now, then you might have to compromise to keep your company floating.