Mercury brings a bit more sanity to Prolog with its mode system. This is one of the key reasons why it runs fast and can be used to build big systems like Prince. That said, it _is_ a restriction on expressiveness of programs, and sometimes that makes it hard to do certain things. That's true of any type system.
There's still an interesting question about whether a different iteration on the mode system could capture more of the semantics of Prolog while still being useful, but so far nobody has done that research yet. It's more trendy to do FP research these days, so I'm expecting it could be a _long_ time before we get an answer, sadly.
