My recommendation would be to outsource for as long as possible and focus your time and energy elsewhere. Build the simplest, least expensive solution possible and test whether there is sufficient marketplace demand for the concept before investing one dime more than necessary.

Cheers, Frank

I would also explore the stated need to allow company employees to use their corporate login. How critical is this to your solution? Could you launch without the integration? Have customers asked for the integration?
There are plenty of b2b SaaS solutions that don't offer this type of integration for variety of reasons. As a startup, you don't have the luxury to go down this rabbit hole if your solution is not dependent on it. Focus on your core solution and integration can be added later.

It is definitely a risk to integrate with any other company. WHY? If that company has a problem your customers will not know or CARE. What your customers will know is your SaaS app was not working.
Now. if you are starting up you have to weigh the time/cost to integrate and build yourself. Risks? Pros/Cons?
If the integration can be done quickly and for low cost you can plan to build your own later and you will also have data telling you if this integration is wanted/needed.

Hope it helps some.