Problem is, often you get an easy integration route, where lib and UI come in one package or a hard route, where you have to build a UI around the lib.
To save time, you use the full package, but then testing becomes a nightmare.
And the sad thing is, the stuff that's hard to test is crucial to test.
I guess in this case what matters is the estimated lenght of the product lifecycle.
I'm right now writing open source library in ts hoping that a ui will pick it up. I may need to write the UI myself at the end, but that will be much easier after I put every feature in my library I can think about :)
I guess your case is something like this as well, that's why it's so hard to test.