Tantalum caps are actually relatively unreliable. They do not tolerate over voltage or excessive ripple current at all. Some types can catch fire when overstressed. Ceramic caps are some of the most reliable caps. And you can find 6.3V rated 2.2uF ceramics in 0402 packages. I have never sen tantalums that small.
Using tantalums is likely ok but we recommend ceramics. As with any switching power supply, each switching cycle, the IC draws its relatively large switching current out of the input cap. This current creates under and over voltages across impedance in the PCB traces and input cap itself. In extreme conditions, this may cause the IC to malfunction, if the voltage gets too low, or fail, if the voltage gets too high. I don't think this will be a problem for this IC, but it is another reason to use ceramics.