Services like these are ultimately short-term solutions. Our office manager was #7 to join and she's been a godsend in helping manage basic HR as the team has grown.
My advice to any founder is to seriously consider making an OM hire sooner rather than later.
Definitely hiring a good OM is a crucial move that lets the founder/manager focus on more important tasks than administrative stuff. However in case of 'hard' HR where up-to-date knowledge of legal, taxes, insurances etc. aspects is required, OM is not enough.
My advice to any founder is to seriously consider making an OM hire sooner rather than later.