SilverStone begs to differ.
Still, it's hard to design fanless units for smaller form factors as they typically don't have much airflow around the PSU housing, which any fanless unit needs. Fanless Flex-ATX would be incompatible with
a lot of Flex-ATX cases simply because they would cover three out of four sides with a steel panel with near zero heat transfer. Same goes for TFX. And frankly a lot of SFX cases too. On the other hand, going the DIY route (with a 12V or 19V AC-DC PSU + a plug-in DC-ATX board like the ones from HDPlex (19V) or Mini-Box (12V)) can net you reasonably powerful yet extremely compact fanless PSU setups.