I'm using the no credit card free trial.
So far, I've completed 1 audiobook and then 4 ebooks, all titles that aren't available at my library or local Overdrive. I have about 2 weeks left, so we'll see if I hit the limits before that.
I downloaded everything for offline use (but only 1 or 2 at a time as I finished, not all at once), and had no issues on a phone in airplane mode with no sim card. The app remembered my position in between sessions when I closed the app; no bookmarking required.
One annoying thing, is that even though I didn't use their app or website 'save' function to bookmark potential reads (I was actually just bookmarking in my browser), even just searching for authors/titles seemed to make the algorithm hit the 'unavailable' on one particular author/title when I went back to view it a second time.
This was with only the audiobook downloaded, too. I think I looked up about 10 titles when it happened.