My personal experience with FlixBus was good. My trips departure on time (even some minutes earlier) and met the approximate arrival time. The arrival time depends not only on the driver. The road conditions, traffic, weather will influence that.
Anyway, when traveling by bus (or car) you should not think you will make it on time and plan to get a thigh connection to another transport. My rule for planes is to have a gap of minimum 3 hours between them. With buses, I increase this buffer.
When there is a delay, you can see it directly on the app. Also, they message you.
That is not helpful if you are waiting at the bus stop, and you do not have an Internet connection, of course.