See, I find that some artists (notably Whedon and Moffat) tend to write really well when given a ton of restrictions, but write worse the more freedom they're given.

I'd agree with that assessment. Having watched both being reigned in a bit, and both not.

They are both very good dialogue writers, and excellent episode writers...but sloppy long-term plotters, and show-runners. They need someone to reign them in a bit or keep them focused.

Although I'd say Moffat's better at plot-twists and tricky plotting and Whedon is better at emotional character arcs (in that way Whedon has more in common with Davies.)

I'm not sure I'd agree that RT Davies is a good show-runner, he reminded a lot of Whedon in some ways. I liked some of his individual stories, but his character arcs..he lost me.
But I do agree about Moffat and Whedon.