dropuser is a wrapper around the SQL command DROP ROLE (DROP_ROLE(7)). There is no effective difference between dropping users via this utility and via other methods for accessing the server.
This utility, like most other PostgreSQL utilities, also uses the environment variables supported by libpq (see Section 31.14, lqEnvironment Variablesrq, in the documentation).
In case of difficulty, see DROP ROLE (DROP_ROLE(7)) and psql(1) for discussions of potential problems and error messages. The database server must be running at the targeted host. Also, any default connection settings and environment variables used by the libpq front-end library will apply.