Most databases (at least PostgreSQL & H2) do have 'concat' function built-in but the hibernate dialects register it as regular string concatenation using || operator.

{{registerFunction( "concat", new VarArgsSQLFunction( StandardBasicTypes.STRING, "(","||",")" ) ); }}

The problem is the two behaves differently when passing NULL values. Built-in function skip NULLs when concatenating and returns NULL only if ALL parameters are NULLs, while the way hibernate translates it returns NULL if ANY of the parameters is NULL.