database sharding, automatic or manual? dont really know which is better.

were hitting some scaling limits with our main user database and need to start thinking about sharding. I've seen solutions for automatic sharding based on certain keys, but it always feels a bit like black magic. On the other hand, manual sharding seems like a ton of operational overhead. What's your experience with sharding strategies? Do you prefer the automation, or is the control of manual sharding worth the extra work?