As someone who has worked on many-TB-sized "custom" sharded systems with 30-150 shards at multiple (ok, 2) employers, a key challenge to the overall sharding landscape is unsharding all the data back at the analytics layer.

This at a minimum often involved adding back a shard key to the physical data, or partitioning, and/or physical data sorting easily in the "OLAP" layer. And a surprising number of CDC and ETL toolkits don't make it easy to parameterize a single code/configuration base, nor handle situations like shards being down at different times for maintenance or fetching data from each shard at a time of day specified by its end-of-day or handling retransmissions or reconciliation or gaps or data quality of a single shard when back in an unsharded landscape. SQL UNION ALL to reunite shards works, until it doesn't.

YMMV but would be curious if you have a story/solution/thoughts along these lines. It's easier if you shard with unified analytics/reporting in mind on day one of a sharded system design, but in the worlds I've lived in, nobody ever does. But maybe you could.