Short answer, it’s actually not.
The actual danger is overloading a socket. This is significantly easier with a chain of extensions.
Resistance on the individual plugs shouldn’t be an actual issue. The only way it might come into play is if you are using a switch mode power supply at the end. This would adjust its current draw to account the reduced voltage. This would increase the current draw at the plug.
E.g. a PSU trying to draw 1000W. At 120V this would be 8.33A. If you drop 30V, then the supply would draw 11.1A. The intermediate plugs would dissipate 333W. You would be drawing 1333W from the wall.
In countries with low AC voltages, and a lack of fused plugs, this could overload a socket and cause a fire.
It’s also worth noting that long extensions have the same issue though it’s less. They also have a far lower rating when coiled than when uncoiled. E.g. I have a 13A extension that can only provide 4A when fully cooled.
I’m UK based, so all our plugs are fused. We also work at 240VAC.
As for my extension, it has no limiter on current, other than the 13A fuse. It does have a thermal cutoff built into it’s reel however. It might not be enough to stop the cable getting melty, but it should stop it catching fire. It would require a very high load, for a long time to reach that point however.
As I said, multi adapters aren’t inherently dangerous. They just make it easier to create a dangerous situation, if used inappropriately.