What do you think about prepared statements? Does HarperDB support that?
MongoDB query language does not support prepared statements, and sometimes fails with strings that starts with $. I tried to wrap in { $literal: "$CONTENT" }, but it is confusing when I can or cannot wrap in $literal.
Also, you probably also realize that NoSQL is somewhat a misnomer, but actually "non-relational". It may use a query language that may or may not resemble SQL. Sometimes, both SQL-like and SQL-unlike syntaxes are allowed.
You didn't even mention much about NewSQL.