SQL databases are very powerful and capable tools, but they also hide some complexities behind a declarative language. That makes easy to get into a bad place, performance-wise, if the DB mutates quickly. You don't need millions of accesses to the DB to have unbearable slow queries if schemas are not carefully designed, which is difficult in early stages of development.

I usually consider the "I had to migrate from DB X to DB Y" a success story. You discovered what was required and later in the process, improved your system. Painful, but probably unavoidable.

Databases are not easy to handle and operate. Is easy to underestimate them. We used to have people just devoted to them (can you remember DBAs?). It is worth to spend time learning them and knowing their goods and bads. And sometimes making mistakes in unavoidable...