Yeah largely sized tube elements combined with lots of gain is a challenge. It’s also hard for those flimsy mica spacers to tightly hold down the grid/plates/cathode spatial relationships enough versus the gain factor, especially after all these decades. Ken Rads in particular often have a nasty little audible internal rattle you can excite with a tap to the glass (this is with the tube in your hand, not plugged in). Those tubes are REALLY gonna give you a headache in preamps.
Even in 12AX7 the short plate variants are sometimes thought to be less susceptible to microphonics. 12BH7 as a 12AU7 sub is another example where the larger plate structure (12BH7) leads to big headaches with microphony.
Later versions of 6SN7 had better bracing and plate designs to mitigate these issues.