The main issues with PM4A as an actual released product:
- Although using Gecko as a rendering back-end, the application itself is separate and written in an odd-one-out language that needs language skills we don't have in-house: java.
- Android as a platform is rapidly evolving and maintaining compatibility with the full range (especially Lollipop+) is beyond the manpower available. If you look at, for example, bug #1030897 - the full Mozilla/mobile team has trouble keeping up with that, not something we can ever hope to match.
- Mozilla's focus on the mobile platform would have them much more in their element than Pale Moon. And I'd rather not continue to have an "almost-acceptable" browser on there that would have to just hobble along trying to keep up, instead of an actual good product with a clear, defined presence.