I can't really answer the question with any insider knowledge, but from experience working with the Map Maker product and Google reviewers (those are the ones with G), they use a contracting workforce for local that has little training and high turnover. The Google Reviewers on the Map Maker team usually have better training and retention, and have higher quality standards that they adhere to, but that may be my bias working with them (they are usually quicker to fix their mistakes, and they also QC the LE reviews, frequently (100%) overturning their reviews and approving your edit on MM if the LE denies your edit. It's very frustrating and almost bipolar.

It's all fairly easy to spam Google Maps from other avenues. I've seen bulk uploads of 100's of spam listings, the Google My Business product is also easily gamed, and the LEs don't appear to closely scrutinize locations in order to ensure that the business is where it's claimed to be, even though they have access to more detailed street view imagery and presumably can just Google it. Google in general, and LEs in particular, seem to have a policy of preserving all data, no matter how bad/spammy, and approving new POIs, even if the POI is of a questionable nature and isn't supported by the facts, which, again, points to a deficit in their training in being able to identify spam features for Maps. Once you know what spam is, it's fairly easy to identify, and the actual QC checks, primarily using government licensing databases and other resources, takes about 30 seconds or less to verify listings. The hard part, at least on my end, is removing it. Google loves to hang on to spam.