It is a well known fact that DBZ power levels are subject to immense scrutiny from everybody. There has been a constant dispute regarding the durability of DBZ characters. A recurring argument which is regularly brought up is the fact that DBZ characters die because of destruction of planets, and as such their durability stat should be changed to planet level.
This theory which explains why DBZ characters are defeated by planet-busting attacks, despite having much higher durability. If there are any disagreements, please state them in the comments section, along with logical reasons to back up your contradictory opinion.
A biological weakness is something which is present solely due to the genetic make of an organism. Most races in DBZ require terrestrial conditions to live and cannot survive in vacuum. DBZ characters are beaten by planet-busting attacks because planet-busting takes advantage of a biological weakness of most DBZ characters (eg: Saiyans, Namekians, Humans, etc.). By eliminating a planet (and by extension, it's atmosphere), DBZ characters die due to exposure to vacuum, and not due to the attack potency of the blast itself.
An example of a biological weakness from another fictional verse would be Kryptonite, the infamous radioactive remains of Superman's home planet Krypton. By logic, it seems incomprehensible that someone like Superman would lose to a piece of rock. Yet he has shown vulnerability and far, far lower durability in the presence of Kryptonite. Again, this is a biological weakness exclusive to a Kryptonian. That does not, however, take away from Superman's durability.
One might think of a biological weakness as the inverse of a hax ability. A hax ability allows the user of the ability to bypass the rules of logic and eliminate people with much higher durability. On the other hand, a biological weakness also makes the character bypass the rules of logic and succumb to attacks of much weaker attack potency.
Resistance to physical factors
Another query which comes to mind is, "What about heat and other such physical factors?"
True, DBZ characters have shown to be concerned about heat. However, I don't believe it is an inconsistency.
As the manga has already shown, DBZ characters can use ki to amp their physical condition (e.g. Goku fighting against Future Trunks' sword with his finger). This suggests that physical factors such as heat, while threatening, are not as much of a weakness to DBZ characters as they can be resisted to some degree using ki.
Planet-busting on the other hand is a completely different ball game since it leaves DBZ characters in vacuum. Theoretically, the characters should burst from the difference of pressure within their body and the outside, but they might prevent it by enhancing resistance to pressure difference using ki. However, there is still a lack of air, which will hamper biological processes necessary for sustenance.
So it is not the presence of physical factors, but the absence of them which makes DBZ characters helpless (because ki can not compensate for what is absent altogether).
Simply put, majority of the DBZ races (humans. saiyans, namekians, etc.) cannot breath in space. Certain characters which do not require air for sustenance such as Frieza, Cell, Buu, Beerus, Whis can easily withstand the destruction of a planet. Even those who do require air (the Z fighters), have depicted being able to resist planet busting attacks.
The durability of DBZ characters is indeed far higher than planet level.
In the movie Battle of Gods, Goku in Super Saiyan God form was shown to be fighting with Beerus in the upper reaches of the atmosphere, where there would be little to no air.
This could be possible because:
1) Inconsistency: The most likely explanation is that this detail was overlooked.
2) Godly ki: It could be that the Godly ki allows Goku to fight in space
3) Short duration of exposure: It could simply be that Goku was in the upper atmosphere for a brief period of time and hence was able to cope with the reduced oxygen by holding his breath.