As mentioned by numerous others, frequency band support is the main issue. But my opinion is that the problem has actually gotten worse
, not better. At least in normal people's ability to figuring things out.
CDMA vs GSM/HSPA/LTE is a more well-known problem, let me explain the GSM-lineage side, and let's only look at 3 main regions: US/Canada, Europe(-ish), and Asia.
Back in the days of 2G, there were essentially 4 frequencies and each region essentially takes on a pair: US/Can - 850/1900MHz, Euro/Asia 900/1800MHz. That's why importing phones from across the pond was a fool's errand because none of the bands would match and your phone is basically useless for making calls/data. The advent of tri-band phones made importing/roaming possible, but still no realistic "global" phone. You always have to compromise because on "non-native" region you would always be missing a frequency band and coverage suffered.
Then come 3G (UMTS/HSPA), these were the 4 bands, and then later 1 more joined the group: US/Can 850/1900 (later 1700), Euro 900/2100, Asia 2100 (some territories). Tri-, quad-, and penta-band phones finally made "global phone" a decent reality. But of course, governments and carriers always had to make things difficult for us consumers.
With 4G/LTE (forget US Carriers' use of the term "4G", I am using the more technical naming convention), there are now 40+ bands (with more being available over time, big money with spectrum auctions) in play, each occupying like 20-40MHz of spectrum space. The Megahurtz number does not work anymore; for example, there are 3 (or 4?) bands associated with 700MHz. Even I can't keep up these days and always have to look things up. In US, for ATT, according to (frequencycheck.com), uses 2, 4, 17, 30. Most urban areas would be 2 and 4. T-mobile is like 4 and 12 in most of their earlier deployed areas. Verizon has that critical band 13. Apple sort of led the way with compatibility by first bringing frequency support for like 10+ bands, but with so many numbers floating around you need to fact check like a hawk to make sure that your carrier's bands are supported when you import phones. Missing 13 has always been a problem for people using Verizon, for example.
The reasons are technical as much as business/political. A radio/modem chip is going to be more complicated, larger, and more power hungry if it supports more bands. Carriers/shops/govts obviously do not prefer people buying phones from other countries circumventing markups and taxes. Even most modem chips bands are "firmware" controlled these days, I don't think phone makers necessarily allow you to just flash some baseband firmware and be able to switch bands. The flashing scene is much more difficult these days. You want to talk about Android fragmentation? LTE bands is a worse **** hot mess!
Kind of back to the original topic. Support and maintenance is another important topic. Apart from Google itself with their Pixels, Nokia, or OnePlus, counting on vendors to give you at least security updates is like begging for some huge favour. Larger vendors with carrier ties, like Samsung and LG, have now become a bit better since the 2-year thing is pretty well established. Even so beyond 2 years don't count on getting any updates at all. Chinese vendors are even worse, especially outside of China. You can basically expect to get no updates at all even your initial damage is small. Whether security updates (forget feature updates) are important, will be up to you. If you do any online banking or shopping on the phone, beware.
So TL;DR with respect to cheap import Chinese phones in US/Canada? Practically a no, even before spying allegations are considered.
So what other cheap phones to look? It won't be like sub-$100, but I would say Motorola and (somewhat reluctantly) BLU may be your ticket. They have decent choices around the $200-300 mark. Updates is still an issue but that's basically a common Android vendor problem. Or, go with those low end Samsung phones that may be 1-2 generations out, the J's and stuff. They would be usable with guaranteed carrier support as most are sold by them. Don't expect to play the latest games on them or decent camera quality though. I recently found out that OIS is a premium feature and had trouble finding a model with it in the <$400 range, unless you dip into the used market.
The Model M is not for the faint of heart. You either like them or hate them.
Gerbils unite! Fold for UnitedGerbilNation, team 2630.