The general picture I get is that much of what is said about the Planck length (and the associated Planck units) is either speculation or outright false. However, one claim that keeps popping up in various forms (including in answers from both of my links) is that we don't know how to describe physics at a scale smaller than the Planck length.
People typically say this in two different ways.
One way is that there's something inherent in quantum gravity theories that makes it impossible to talk about distances lower than the Planck length. Is this true? And if so, what makes us believe this is true? According to wikipedia, the length of strings in string theory are on the order of the Planck length. But I don't know anything about string theory so I don't know the implications of that.
A second way is that the Planck length is the scale at which gravitational effects and quantum effects start being comparable in which case our current theories (quantum physics and general relativity) clash and we don't know how to describe what is happening. Is this true? And if so, what makes us believe this is true?
One argument I've heard for this second interpretation is that the Planck length contains G, c, and h bar, constants from quantum physics and GR, and thus when this is 1 both quantum and relativistic effects are important. However, this argument is incredibly dubious because this exact same argument could be made for a length equal to any constant times the Planck length or for the Planck mass, which is clearly not in any sense a limit. Is there some better more rigorous way to make this argument? Perhaps by looking at some well-known system and showing that GR and quantum effects are comparable exactly at the Planck length?
In sum I'm trying to get a better handle on what the Planck length really means. Any help would be appreciated.