Fortnite at 60fps?

Can you build very well in Fortnite at 60fps?

Yes you can.

However, this has a huge disadvantage compared to other players who achieve higher frame rates.

I always play at 60 FPS and (from my point of view) I'm quite good.

With more FPS, more is certainly possible, but it is possible

Take a look at Aunt CHianti and she plays very little FPS on mobile and can still build better than some PC players

The disadvantage is not huge.

I have a buddy who has 60 FPS and pulls some with 120 FPS

Yes, the disadvantage is very big, especially in competitive, I speak from my own experience.