What is an "international model" iPhone?

The difference is basically which cellular frequencies they use. An international model (or “world phone”) uses the GSM standard and so support all four major GSM frequency bands. I don’t know what problem you may have read about, but one complaint I have heard is that these phones go a lot slower on LTE networks.

For problems, the only thing I’ve read have been warnings about Asian ripoffs which look exactly like iphones (and are sold as iphones) containing tons of malware/spyware.

So, I would imagine that could be problematic because, unless one is purchasing from a trusted and reliable source, how could you tell the difference?

Obviously if you’re getting your phone from a major carrier that’s unlikely problematic, but if buying on ebay? That could be dicey.

Not only eBay, but many phones offered for sale on Amazon are described as “international models”.