Is it safe to use my US iPhone/iPad charger with international plug adapter?
The short answer to your question, "is it safe," you kind of answered yourself.  It depends.

Is it technically possible? Yes. My Apple- (and non-Apple-) branded adapters show a range of 100-240VAC, 50-60Hz. I've used them as-is in the US and Europe without issue. While you should be "just fine," don't just take my word for it. Read the labels on the power supply.

Generally, when the voltage drops, the powered device (or the drawing side of the power supply itself) makes up for the difference by drawing more current. (I've seen this numerous times on 48VDC-powered systems: when the voltage drops much below 44V, circuit breakers trip and/or fuses blow because the current exceeds the rated amounts.) Without a breaker or fuse, that excess current causes elements inside the power supply to heat up beyond tolerances and eventually burn up.  Surges in voltage can overrun the semiconductors as well, with a similar result.

Special shut-off circuitry in some power supplies is supposed to kick in, protecting the adapter and the powered device. Cheaper adapters may not have this kind of protection, or they become the protection for the device by sacrificing themselves. 

I would bet that in more developed/commercial areas where the power is more trustworthy, an adapter will be fine. For extended use cases, like a laptop plugged in for a number of hours while in use, especially in sketchy areas, you might want to invest in something that helps smooth out the voltage (surge suppressor, for example) or the transformer/regulator you mentioned.