It seems that reversed geometry or low inductance ceramic chip capacitors (MLCCs) are simply better in every spec than regular geometry, and may allow using just one value to bypass instead of two (eg a reversed 1uF instead of a 1uF and 0.1uF). Despite that, they seem to be a specialty/unusual part (for example, Digikey stocks 6M of the most common 1uF 0402 but only 60k of the most common 0204).
Why is that? Are there any hidden drawbacks?
Cost is the only real drawback. All the manf of lower esl packaged caps seem to want to charge you extra for the privilege. I've always assumed they had a meeting along the lines of "one of these caps replaces two regular ones so they must be double the price."
Plus if I recall they're not dramatically better. I could achieve a similar gain by going to 0201 or 0402 so why mess around with more expensive parts. If I want to be fancy I'll just use some idc parts :).
They're not bad for the values where say you can't get a 0402 or a 0201 but that starts to limit their useful range. But I think bang for your buck wise I would consider an x2y cap before bothering with 0306s much better value per real estate used.
I'd also add that there's much more to decoupling than just 1uF and 0.1uF so I usually prefer a wider range (.1, .22, .47, etc)
There's probably a chicken/egg problem. They are more expensive and less available because no one specs them.
Because they cost more and are harder to get, I will only spec them where required.
It doesn't help that traditional bypass caps are plentiful and practically free.
