Remarkable: I found myself wanting to return to the challenge multiple times, even though sometimes the distribution of balls made success impossible.
Needs Improvement: Movement was frustrating, although it did seem like I could translate horizontal momentum into vertical momentum, which helped. Since your code isn't doing anything directly with physics, you probably shouldn't use FixedUpdate (in the BallSpawn script) (more on that here: https://docs.unity3d.com/Manual/ExecutionOrder.html)
Successful: Finding a gap and speeding through it was satisfying.
General: Simple and well put together. Scripts are well organized and commented - good use of methods. Assets/Scene are well organized. Introduces some simple UI. Overally the game is very simple but it's fun. No complaints.