[Suicide Junkie]

You might be able to nuke a bunch of small and/or undefended colonies to glass in one turn with one massive fleet, but why not? If they have the speed to visit each planet in a month, they can easily drop 50 self-guiding nukes as they zoom past to slaughter each colony.

If there are reasonable defenses, however, your fleet will take some damage. Damage to a single ship can slow the entire fleet to a crawl, and wreck your "smash everything with one stack" plan.
If they have engine-disruptors on a weapons platform, your fleet is almost guaranteed to be unable to attack anything else that turn. Null-space and shield-skipping weapons are also particularily effective.

It is up to the defender to make his territory harder to sweep.