Also an option, when choosing what to use right at the startt, is being careful about using an open source solution from a for-profit startup, and evaluating all your other options, taking into account that it may not remain open source, and if it doesn't, what place it has in your business, how hard it would be to switch then, etc.
Right, the overhead of setting up SAML on your own is _a lot_ and things like this usually come with a Wal-Mart’s worth of foot guns. Even so, I’d be much more keen to spend the time up front diving into it and working on an in-house solution, rather than find myself and my team up a creek with a broken auth solution and several sprints worth of work to fix it, that’s also going to push other work out because logic is critical.