I read in the article a common writting : Schema flexibility is another major differentiating factors between SQL and NoSQL. The relational world imposes a cle

I read in the article a common writting : Schema flexibility is another major differentiating factors between SQL and NoSQL. The relational world imposes a clear schematic structure during the design phase itself and any change in the structure is generally considered expensive. NoSQL offers Schema-on-Read. This makes it a lot easier to change the data format as data evolves. I only worked briefly with a nosql 'real' production database but we needed to change the data shema, and migration was a pain (like any migration). Was it because some special user case ? Is migrating data really simplier with nosql for a reason I don't see ? Naively I find them even worse because you can have multiple shema at once on the same table, so each read on an important document must include a sanity check 'what is the current shema of this particular entry, should we migrate it ?'. What do I miss ?