Yeah, that's been my experience too. The AI tools are great at the straightforward stuff - adding columns, basic index creation - but they bail on anything requiring domain knowledge about your data.
The real bottleneck in migrations isn't the SQL generation, it's understanding the constraints. What does "backfill this column" actually mean for your users. Are there concurrent writes. Do you need dual-write logic first. That's where you need a human thinking through the problem.
I've found these tools most useful as a starting point for documentation or boilerplate, not as a replacement for the migration planning work. But yeah, if you're hoping to hand something off and not think about it, you're going to have a bad time.