If you're smart enough to get a bachelors or masters in a subject, you're smart enough to get a PhD in the subject. What will make or break you is your tenacity and willpower and not giving up when it becomes a dull hard slog.

And realistically the difference in quality/difficulty level of work people do to get their PhD is massive. Some people revolutionize their field for decades to come and some do a slightly more drawn out bachelors level thesis. Yet they both end up with the same degree.