the reliability factor has more to do with the rimfire ammo, as opposed to the firearm. .22lr ammo, even the more "premium" stuff, is notorious for being unreliable, especially is semi-automatic pistols. there are a number of reasons for this, i will try to explain.

the primers are much less reliable than any centerfire rounds, and any rimmed ammo has a much greater chance to produce a FTF(failure to feed) or an FTE(failure to eject) simply because the rim is more likely to catch or get hung up, as opposed to the bottlenecked design much more common to other semi-auto loads. these problems are amplified when low powered rounds are added to the equation. you don't run into those same problems with a bolt or lever action rifle in .22lr, as the rounds are chambered manually. revolvers are a different ballgame because of the revolving cylinder, thus no feed issues which is one of the many reasons they are renowned for their reliabilty.

there are a number of revolvers chambered for .22lr, but a suppressed revolver is extremely uncommon and expensive due to the fact that you would need a custom made threaded barrel, as well as something to muffle the sound caused by the gas escaping from the cylinder gap, and its highly unlikely that a hitman will go to all that trouble for a throwaway gun.

.22lr ammo also has the problem of often being mass produced so cheaply, and on such a huge scale, that weak rounds are common, which again will fail to cycle the action properly. the same problems exist for the subsonic loads used in suppressed weapons, as the are naturally less powerful to achieve the sub-sonic velocity needed to lesson the sound.