I do wish this wasn't a sponsored ad masquerading as an engineering blog until the last paragraph, but yes, these things are extremely challenging. What people miss is that the big cost isn't building or integrating these things, it's supporting them. I was the IAM features product manager for a mid-market B2B SaaS and had to jump on calls multiple times a week to help our Fortune 500 client base with planning our their deployment of directory integration (SCIM). You would think you could train support staff for the task, but with each IdP having its own zany behaviors and nuances (or just not following the protocols at all,
cough Azure AD), you end up needing to pay someone who is extraordinarily technical to be an absolute authority in how these things work and are managed at both ends of the connection. I was backstopped by two other support engineers who could help with the day-to-day and escalated to me as needed. So multiple staff level engineers, all just to support two identity related features (SSO, SCIM) for one mid-market SaaS.
Everyone likes to gripe (including here in this thread!) about B2B charging for Enterprise features like SSO, but withhold your judgement until you've actually lived through supporting it. The burden of helping a company like Meta integrate their weird, bespoke, homegrown identity solution into your business app could literally cost your business tens of thousands of dollars per year (and no, that's not a made up example).