You know the gag. Someone is at a crowded party, or a construction site, or at a concert, and trying to impart a private bit of information in an increasingly loud stage whisper. Inevitably, the surrounding volume suddenly drops -- right at the moment where it will cause the most embarrassment.
Well, sound effects fall prey to this.
The dynamics of a musical, particularly, mean that playing an effect one second later (or one second sooner) will mean it is either completely lost, or inappropriately loud. Often enough the same effect can be both...if it lasts long enough, that is.
Sound effects are so exquisitely sensitive to context anyhow. Played in rehearsal, an effect may sound brittle, over-bright, and far too loud to be realistic. Played in the first dress with full orchestra, the same sound is suddenly muffled, and way too soft to be believable.
Of course neither conductors nor directors have any grasp of this. The conductor will randomly add a new keyboard part or a drum riff right in the middle of your cue. Meanwhile, the director will insist on setting levels...and everyone who isn't a sound designer will go around thinking that it is possible to set one single level and it will just work, all the time.
A live band can flex. So can an actor. Their performance dynamics alter as the audience noise, the acoustics of the space, and so forth change around them. Pre-recorded sound effects don't flex the same way. Their volume does not depend on audience energy or room temperature but is set by electronics.
The only save a canny sound designer can make is to put a real-time fader on background sound effects. That way, they can be dialed in from moment-to-moment with the same front-of-house mixing that is applied to singers and orchestra. But even that won't save spot effects; one can't react fast enough. So from day to day, performance to performance, and within a single performance even, the same effect will be at the wrong volume as often as it is the right volume.
This is particularly an issue around certain climactic effects. Sometimes you have an effect, like a gunshot or a guillotine, that is in the middle of bombastic music or in a moment the ensemble is reacting to. Their screams (or the orchestra's kettle drums and keyboard brass) will cover up your sound unless it is played 20 or more db higher than is otherwise called for.
And, of course, it is inevitable that the ensemble will decide to scream at a slightly different moment, or the band hit the big drum flourish in a slightly different moment. Less than a second off is all it takes...and what was the appropriate gunshot becomes a ludicrously, painfully, loud bad-sounding effect that will make everyone turn in their seats as the poor designer shrinks in embarrassment.