I'm sure ArangoDB is wonderful technology but your characterisation of MongoDB is misplaced. The people who build the WiredTiger storage engine that MongoDB depends on have been building database engines for over 20 years. They created the original BerkeleyDB, built Oracle's NoSQL engine then left Oracle to create the Wired Tiger engine. They were acquired by MongoDB to completely revamp our storage engine technology in 2014. Don't let 7 year old memes cloud your judgement.

(I work for MongoDB).