My understanding of the basic operating principle of switchmode power supplies is this:
* The input voltage is rectified and used to operate an oscillator
* The oscillator output is fed into the primary of a transformer, with the secondary producing the target voltage
* The operating point of the oscillator is controlled to generate and regulate the target voltage (via feedback)
* As such, when the input voltage is low, the oscillator will be driven harder, drawing more current so that it can deliver the required target voltage at the current required.
My (simplified) interpretation of this is something along the following lines...
If a SMPS requires 120VA to be able to deliver its target voltage (at the requisite current), then it will draw 1A from a supply voltage of 120V. If, however, the supply voltage is 8V, it will try to draw 15A.
This level of current is likely going to be a big problem for a circuit that was designed for 1A typical.
But this is just a fundamental issue. In addition to this, you will have the practical issues of the ability of the actual circuit to operate in such a situation.
On that basis, I would not be presenting any SMPS with notably low source voltages - unless it was a design meant to work with them.