As well as the above points,
Unless I am mistaken Roon doesn’t handle picking up from where you left off or ID3v2 chapters markers.
Roon will also mix your spoken audio files in with your main listening stats and given their length will therefore skew your listening stats on the home page.
Similar issues also also affect Podcasts and long form audio content like DJ mixes and archived radio shows.
So, while you can use Roon for audio books the current user experience is severely lacking compared to a dedicated audio book app.
Whether Roon should improve its feature set there is, like Podcasts and other non album centric audio content, another question.