1: How many megatons do you feel would equal a punch from Hulk, Kurse or Thanos? How powerful are the people in Hulk's weightclass? Class 100? Mountain busters? Moon movers? City busters?
2: Why do you feel a bit of space debris invalidates the obvious continent busting+ forces one would have exert to eject said debris off of the dying, exploding fireball that used to be a planet?
3: The single reactor pulse that the Deathstar used on Jeddha wasnt even remotely busting the planet. It was a mere "extinction level event". How well do you feel Thor would handle being 1 kilometer away from its blast origin? You can find it easily on YouTube.
The Bifrost has been shown to be summoned in multiple methods over the years and by multiple operators. Can Heimdall bust a planet? He can "summon" it. How about Odin? He used dark matter to "summon" it. If the Bifrost is REALLY just the Bifrost, certainly they are all planet killers?
My biggest point here is that bullets are simple concepts but the quality of the "gun" matters.