Cassandra has very lightweight transactions.

What he lists as his transaction use case (update if unmodified) I do right now with Cassandra and an IF clause and a timestamp...

i.e. update foo set x = y AND last_modified_timestamp = 456789 IF last_modified_timestamp = 12345

see http://www.datastax.com/dev/blog/lightweight-transactions-in...

The mapping tables are used to store relationships between objects, not to define shard locations. The shard mapping is encoded in the object id.

In any case, I would have a knee jerk reaction to not trust any database system that hit 1.0 a month before I wanted to start using it, especially for mission critical core data.

Technical debt doesn't necessarily arise from using the wrong (a.k.a pretty similar but with a few features less than what you need) tool. In my experience, it's more likely that technical debt arises from trying to use thew newest tool (which looks to be a silver bullet) rather than trusting your own experiences. It's surprising to me that people are willing to move to a completely different technology rather than extend and repurpose what your engineers are already familiar with.

But if you also need to use MySql anyway, Cassandra would be the "wrong Tool". Because you would have to maintain two DBMS instead of one. You want to minimize complexity and this can mean implementing your own Sharding.