This is why Gary Gygax didn't like critical hit/fumble systems. His argument was that while any individual monster has a small chance of being affected, the PC's, because they are in many, many battles, will eventually roll a critical fumble or be subject to a critical hit from a monster. Any group using the really nasty critical tables, such as losing limbs, permanent scars, etc. were guaranteeing that the PC's would be subject to such effects at some point.
But most players like critical hits, so 5E kept them. They've limited the effects to additional damage, but it still means a PC can die outright to a crit. One of the risks of the game, if using RAW.
So long as the encounter is within the bounds of what the party might handle, this isn't a problem. Take a party of four 4th level characters:
- They face a single hill giant that does 3d8+5 with a greatclub, 6d8+5 on a crit. That's a lot of damage, but what are the odds that it will crit during the entire fight. Not very good, as it's only a single giant with two attacks per round.
- They face several goblins (not sure how many, say a dozen or more). Lots of chances for crits, but the goblins only do 1d6+2 damage, 2d6+2 on a crit. An average fight might have one or two crits, but even if the goblins are on a hot streak, they'd have to land a lot of crits, mostly on the same character, to add any significant risk of PC death.
So small numbers of creatures doing a lot of damage will rarely crit, but when they do...ouch! Large numbers of weak creatures will crit more often, but when they do...shrug.