I fear not the man who has practiced 10,000 kicks once, but I fear the man who has practiced one kick 10,000 times.
-- Bruce Lee
I just finished a guitar practice session and realised to my disappointment that some of my fundamental skills are not as sharp as they used to be. I focused on writing songs or on technique flourishes and neglected the core techniques. I play metal electric guitar, so there are just a few techniques in my opinion that you need to nail. I need to start practicing them daily again.
This is applicable to a lot of different fields as well. From what I've studied, most of the masters in various disciplines have a very deep mastery of the fundamentals instead of spreading thinly. But of course this is if we're talking about a single discipline. There is a lot of value in studying multiple discipline and combining ideas from them into something new. But that's another subject for another time.
Have a great day!