Ah, unbalanced shards via wrong sharding keys was an issue at one point, IIRC. I remember talking with an SRE there when something bad happened at GitHub last year, and I know that this time the current DB cluster failed.
To be clear, I _was_ mapping previous incidents with this year's incident — no competitor or hard feelings involved. I really like Vitess, fwiw. And the only thing I really love is FoundationDB :)