Well, in the scenario I envision, the government dismisses voluntarily to avoid going to trial on wrong jury instructions. The government doesn't have unfettered discretion to do that, it needs court permission. And, in some cases, the dismissal might be with prejudice (meaning can't be re-indicted). So this would be a last ditch strategery. Classically, "jeopardy" does not attach until the jury is sworn, meaning up until then, usually, any dismissal can be re-indicted.
There's all kinds of dismissal types and outcomes, mistrials, voluntary, prosecutor misconduct, blah blah blah.
The other possibility in the event of terrible instructions is a mandamus appeal, which is a kind of rare deal that I'm not even sure is available.
Bad jury instructions, usually in a subtle way, are common in civil cases and a fruitful area for appeal. When you get stuck with shitty ones and you're the government in a criminal case, I'm not sure what you do.