Projections for gamification in a social app

We have this company-internal social app where ppl. get 3 random cards with social tasks they need to finish to earn points, they can also reject a card once a week. For gamification/envolvement/fun we have introduced "achievements", for some of them the players get extra points. E.g.: "first card of a deck" (cards are organized into different decks), 3rd task ever, 8th task ever, complete deck, ....

Interesting parts, apart from how cool projections are to create any sort of metric you invented while the app is already live, could be:

• how we implement that with EventStore DB with sucscriptions per "achievement" where we store commit checkpoints
• issues/open questions we just ran into which are based on the design at a point in time we did not know about "achievements" (not everything is possible/simple with ES)
• how much sanity checks, e.g. order of events, to do for such "view model" projections aka. how much should we trust our events?

Prerequisites

Basic idea about ES and projections.