MU is sketchy on adding releases. Releases need a registered group, and groups are required to have websites, and need to be registered first before releases can be added. batoto is supposed to be an exception to this rule, in that releases only on batoto are allowed, but you still need to register a group first by creating a group on batoto, linking it to the release, reporting it to MU, and only then can you submit a release. By this time the rules are complex enough that people don't bother putting them on MU, or sometimes there are long delays where releases show up a long time later but seem to have been submitted early on (?), or whatever.
A combination of three sources is usually good:
1. MU as main source of info
2. batoto for releases, usually by 4chan, which tend to show up on MU weeks or months later. the quantity of these releases is low, but they're usually of series that are of higher quality than average, so batoto is much more useful than you would expect
3. mangapark for licensed stuff (i.e. rips of English books such as Planetes or Aventura) that shows up nowhere else, as a last resort. also for groups that are "illegitimate" because MU rejects a lot of crap, and batoto loves "official" scanlation groups. example: Ruler of the Land, although the junk releases now show up on MU also (after half a year)