Absolutely not true. Lack of mosfets don't simply go boom, they have built in protection mechanisms to prevent overload, by cutting back on power delivered, causing instability or throttling.
My mITX Z68 MB was one of the first mITX board available with Z68 for OC, but it only had several phases and every single user (check out [H] forum on SFF rigs) on this board experience throttling of CPU OC above 4.3ghz for i5 and above 4.1ghz for i7s. It didnt even come with heatsinks so as soon as it gets too hot, it loses efficiency and throttles more. I had to install zalman copper mosfet sinks on them to stop throttling above 4.3ghz.
As soon as the next iteration came out with more mosfets and heatsinks on them, mITX owners could push the OC as high as ATX.
Edit: There is a direct relationship for mosfet operations, higher temps lower their efficiency causing more input required per output. Each mosfet has a maximum current, hotter mosfets in combination with not enough, would therefore cause throttling, or for gimped no-safety boards, pop!