Can a fixed output buck have too much output capacitance?
I believe, and someone can correct me if I am wrong, that a buck converter has a natural tendency to oscillate, and that proper feedback counters this, but larger LC aggravates it.
To illustrate my point, consider a buck converter as a square wave generator followed by an LC filter, followed by a load.



simulate this circuit &ndash; Schematic created using CircuitLab
Here the square wave generator is &quot;ideal&quot;. It sources or sinks current at any voltage, with 0 output impedance. There is no diode loss, etc. The frequency is 100kHz, and it the output is either 0-5 volts. Since it has a duty cycle of 50%. So it has a DC component of 2.5V. There is no feedback. Proper filtering should remove most of the AC leaving a steady 2.5V for the output.. According to the simulation it comes very close to that, except at start up, and a few &quot;glitches&quot;.

Again, there is no feedback at this point.
Now, if we change the filter capacitor from 220nF to 2.2uF, we can see that the output is much less stable.

And, if we make the capacitor even larger, 22uF, we get this:

Adding capacitance decreases the square wave frequency ripple, but it makes the LC circuit more susceptible to longer term ringing at the LC resonant frequency.
Using feedback to adjust the duty cycle of the square wave may compensate for the tendency for the LC components to ring. But higher C creates a higher burden for the feedback loop.
As mentioned, this uses an ideal square wave generator. An actual buck converter does not have symmetrical characteristics at the high and low parts of its duty cycle. My experience, is that this compounds the problem of ringing, and one can get a sustained ring that never decays, even with a fixed duty cycle. In the 5V to 2.5 volt buck converter described, if the square wave generator is replaced by actual components like an FET and Schottky diode, the sustained ringing (never decays) may be as high as 100mV when using a fixed duty cycle.
Edit: I ran some simulations with a Schottky diode and P-channel Mosfet. I did not see the sustained ripple I have seen before. However, again, the smaller capacitor had more ripple, but better transient response. Here is circuit I used.



simulate this circuit
With a 220nF capacitor, I had this output.

When the capacitance was increased to 22uF, I saw a much higher peak transient error! (But not the sustained ringing I had described).

I am wondering whether the sustained ringing that I recollect was with feedback, rather than a fixed duty cycle. Although I don't at the moment know how to reproduce that sustained ringing, it is something you may want to watch out for. Perhaps someone else has seen it, and can report.
Hope this gives you insight into some problems associated with excessive filter capacitance.