As far as the OP goes, there is indeed no need to worry as long as the transformer is not underrated. I have two of these things in my house, they're $65 autotransformers but hugely overrated for their loads and will basically never fail. You can spend the extra for an isolated transformer as linked on the first page but the additional cost is so huge (as much as a new flash!) as to be pointless.
If the voltage ratio is greater than 2:1 (e.g. 240V -> 60V), then there is more current through the parallel winding (N-1 times the input current for a voltage ratio of N) than the series winding, so it's the parallel winding that will fail. If we're talking about a 2:1 transformer that's overloaded, there's still a 50:50 current ratio through the windings and a 50:50 chance of the catastrophic failure. Best idea is to not stress the transformers, which is why buying any old $20 100W autotransformer would be a really bad idea - you run a 50% chance of destroying your flash by plugging it into that.
Fuses prevent fires, not damage; the fuse will only blow when the current gets too high, not the voltage. Especially once you consider fuse reaction times, the load device will certainly be destroyed (though not necessarily irreparably) before the fuse pops. The fuse is depending on fault-current through the load to blow so by definition it cannot save the load from damage. While it is possible to build an over-voltage protection circuit, they're not included in these cheap transformers.
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