Placeholders are enough to prevent injections. You might still be open to buffer overflows, but that is a completely different flavor of attack from an SQL injection (the attack vector would not be SQL syntax but binary). Since the parameters passed will all be escaped properly, there isn’t any way for an attacker to pass data that will be treated like “live” SQL.
You can’t use functions inside placeholders, and you can’t use placeholders as column or table names, because they are escaped and quoted as string literals.
However, if you use parameters as part of a string concatenation inside your dynamic query, you are still vulnerable to injection, because your strings will not be escaped but will be literal. Using other types for parameters (such as integer) is safe.
That said, if you’re using use input to set the value of something like security_level, then someone could just make themselves administrators in your system and have a free-for-all. But that’s just basic input validation, and has nothing to do with SQL injection.