Is it safe to use my US iPhone/iPad charger with international plug adapter? This question has been asked of European adapters, however, I am traveling to some other countries such as India, Thailand, and Indonesia. I wonder if it is safe to charge my MacBook Pro and iPhone 8 simply using an adapter in those countries.

I know most appliances these days are 110/220 dual voltage and I've had no problems charging my MacBook pro in hotel rooms in India before. However, I will be mainly staying in homes during this visit - I did some research and found out that there are commonly neighborhood "brownouts" in India and several other Asian countries which can fry your electronics. I've been trying to research my options and the best I've found is to purchase a voltage transformer with a regulator. I wouldn't even require the transformer aspect and the appliance is both expensive and bulky.

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.