And most genetic "mutations" also come with serious downsides. Myostatin hypertrophy that a subject doesn't outgrow, (it usually manifests in childhood and disappears by late childhood) is accompanied by severe heart muscle damage. Its not just the show muscles that overdevelop, but ALL muscles, including the heart which is in its entirety.
The world isn't some comic book where a chemical, serum, radiation or insect bite creates a muscle freak that lives a normal, healthy life amongst us, except when they rip off their shirt.
You tweak a gene, it doesn't effect just one thing.
EXACTLY. If it were that simple, we'd all be genetically engineered by now. A lot of it is down to gene expression modulation, which is an immensely complex area.