Narrow range!? You think his performances as Worzel Gummidge and The Doctor are similar????
Not that there's anything bad with narrow range. McCoy is my favourite Doctor but I'd admit he generally plays a certain type of character.
No, the Doctor and Worzel Gummidge are not similar. In my opinion, the fact that he was able to play more or less convincingly two different roles, one of them very broad, doesn't mean he didn't have a limited range.
And yes, having a limited range is very bad for an actor. It didn't just mean that he couldn't play a lot of different roles, it also meant that he couldn't play a lot of different emotions, and that's what acting requires. Of course, as you may remember, I think McCoy is an abysmal actor, so I won't add anything, besides saying that at least Pertwee had some gravitas.