Let's suppose I have a circuit which contains a 3.3V MCU and a buck converter (let's think that its frequency is higher than 400 kHz). I'm wondering if shielding the entire circuit would not a good idea because I think that the shield may reflect the noise generated from buck converter to the MCU. Is this a valid concern?
The short answer is that you're right. It is possible (and i mean possible separately from likely) for the shielding of a circuit to cause a problem because a circuit is not self-immune to the quantity of EMI it is generating when not allowed to radiate to free space.
The longer answer is that it's why you often see only select parts of circuits shielded. You might only shield the sensitive parts (like the MCU) to protect it from outside radiated EMI sources. You might equally shield things like switching power supplies to protect the rest of your circuit from them or to protect other circuits from the EMI generated by your supply. If you get problems from conducted EMI then shielding won't help you and conducted EMI may even get worse if you can no longer radiate the energy you're now containing.
The MCU is not sensitive to "noise" as long as you don't have any analog circuit (e.g. ADC, analog comparator) and as long as the noise is not extreme (e.g. next to very strong and fast changing E/M fields). So you problably don't need to shied it from your small DC/DC converter. You just need to take care that the power supply is clean from spikes (some ripple doesn't matter), but that's not accomplished by shielding.
If you want to protect the outside wold from your circuit shielding may make sense.