What effect does switching frequency of voltage converter have on my project?

It shouldn't have any effect since the output is DC (zero Hz).

It will affect the overall design and parts selection, particularly the inductor selection.

And, there's the possibility of RF leakage or noise on the DC line which could be an issue in certain analog applications. If you have an audio application, you'll want to be well above the audio range, and if it's an RF circuit you want to be away from the transmit/receive frequency, etc.