I don't think it's so difficult or unsafe to take my hand away from brake levers for the 1.5 sec it takes to ring a bell and replace my hand (if I take my time). I use this often when cycling on a mixed-use path, as pedestrians I'm passing often will not seem to hear my voice calling out as well as they do a clear bell sound, either because of conversations with each other or walking with earbuds/headphone playing.
I'm comfortable putting on a or taking off a jacket while cycling with both hands off the bars, and eat/drink without stopping on every long ride - so certainly wouldn't say that it's difficult to take your hand off the brake levers to ring a bell. Context and timing matters when it comes to safety though.
As mentioned, the majority of my riding takes place on the road. The times that it's necessary to notify other road users of something (like if they seem about to cut me off), I am invariably in close proximity to a car or truck and thus want to be covering both my brakes with my hands. Removing my hands from the bars in this sort of situation is dangerous, as that 1.5 seconds of time could easily lead to a crash if the car doesn't hear the bell or simply ignores it and I have to swerve or brake hard.
I'd also say that if others do not hear your voice, you are probably not raising it enough. My 9 year old child can easily out yell the bell on his bike. An adult voice is hella louder than a bike bell, and (unlike the bell) can be heard over traffic and engine noise, and usually over music through closed car windows.
For bike path stuff without traffic where things are moving very slowly and it's quiet enough for a little ding sound to notify others of your location - sure, bells serve a purpose. Much friendlier than screaming!
On the road though, bells are pretty useless and often dangerous to use.