The query language is deliberately less expressive than jq's. jsongrep is a search tool, not a transformation tool-- it finds values but doesn't compute new ones. There are no filters, no arithmetic, no string interpolation.
This does make it distinctively less useful. I find quite a lot of the time I need filtering or transformations when doing complex stuff. Which when dealing with larger documents is becomes almost always. So the main benefit, it's speed, does not really matter if I cannot use it for a task. And if the only tasks I can use it on are simpler ones then I don't need its speed and it is not worth the effort to learn or use.
TBH I don't use jq much these days either. I switched to nushell a while back and it has native support for everything jq (and so this tool) can do. But I find it hard more intuitive to use. Every time I touch jq for anything more then just lookups I need to reread the docs to remember what the syntax is. In a much shorter time with nushell I don't need to do that anywhere near as often. Plus it works with yaml, toml and most formats.