SQL Server operations for JSON are parsed from text content on demand. In other databases, the JSON content is broken apart and can be queried/indexed on that parsed/stored content. The closest you can get with SQL is a computed column from the JSON that you can then use to index against, it's very messy by comparison. If you use a lot of JSON queries in SQL server it can get excessively sluggish as well compared to PostgreSQL or even MySQL/MariaDB.

It's definitely a shortcoming of MS-SQL, which iirc is being addressed to an extent in the next release.

There are plenty of points where MS-SQL Server is very nice. JSON support is far from one of them. Even if the JSON is still a step above XML support in SQL Server.