[Quoth]

I've used the Google Play app on Android for some free ebooks that mysteriously don't have downloads. Better than Borrowbox and no worse than Kindle.

I've had no problem with ACSM downloads from Google Playstore Playbooks, but I use Linux and plug-ins on Calibre. Not used ADE for about 10 years.

Considering Google's size and Playstore, their books product is a failure. Any one of Kindle, Kobo, Apple, Smashwords/Draft2Digital, Nook, Tollno, Scribed/Everand, ebooks . com etc seems to sell more ebooks.
We've never sold any via Google.
Partly people don't like the way Google operates payment methods. Hence even on apps the paid sales are tiny compared to "free" (often ad or personal data financed, or distribution of app for a sold HW product). Obviously the Kindle app is the most downloaded ereader app as Amazon has over 90% of English language ebook sales and the majority of ebooks are read on phones.

EDIT:
Some Google content is PDF based and some of that uses DRM. Some paid and free epub content on Google Playstore Playbooks is DRM free, unlike Amazon, who always put DRM on KFX.