I don't agree that he was outright flat, but they do have to cut around some of his less than emphatic deliveries.

The looping was a smart thing, and thought it worked well within the story given his cover.

Given a few more films, he'd have grown into the role nicely, especially in Golden Gun, but you'd have had to recast Christopher Lee perhaps, unless they played him as the sage assassin and Bond as the talented but younger killer.