Distributing the output capacitance of a buck regulator
There could be a downside, but it depends.  The manufacturer's recommendation is usually based on characterization with a given capacitor or range of capacitors.  The safe approach is to stay within those guidelines. Be sure you know if the capacitance specified is effective capacitance, or a specific recommended capacitor.

By deviating from the guidelines you may introduce extra ESR, moving the ESR zero and changing the response of the control loop.  0402 capacitors have worse derating with voltage and can have worse ESR) so the extra parasitics and loss of effective capacitance could affect performance.  

Over temperature an 0402 biased near its voltage rating can lose more than 80% of its capacitance, depending on dielectric and value:





You may also not be satisfied with the output ripple given the extra ESL and ESR you're adding.  You could run some simulations with various stray inductance and ESR values to get a feel for it.

It would likely cause less of an issue with a current mode topology vs. voltage mode, and it might be fine, but you should at least prototype it, and measure the loop and transient response before committing anything to production.

Further info:

An 0402 1uF cap does show higher impedance than an 0805 case size 50V MLC cap-  From Murata's Simsurfing tool here's an 0805 in blue and an 0402 in green:



And here's the derating curve for an 0402 50V 1uF X5R:



And ESR vs. frequency, 0805 in blue again:



So derating might not be a factor for 1.8V rails, but with temperature and tolerance even at 3.3V you're losing significant capacitance.