The Miui OS is hugely popular, for good reason. It has quirks and quality issues like any other phone OS, but it also packages a stock-ish Android experience with a ton of very useful features and a bare minimum of bloat. Updates are regular as clockwork, even on older devices.
Indian and Chinese consumers might want an iPhone or a Galaxy S8, but they're overwhelmingly buying Xiaomi devices. Western brands should be seriously worried about the threat that Xiaomi represent to their fat margins.
You don't have to register a Miui account to turn on developer options - you just tap "Miui version" in the "about phone" dialog seven times. It's a slightly eccentric UI decision, but I understand that it's intended to protect naive users against shooting themselves in the foot. You do need to register a Miui account to unlock the bootloader and install a custom ROM, which isn't an entirely unreasonable security measure.
Mi Mix 2 looks way nicer than iPhone X.
And iPhones are actually late comers for this bezel less trend. Sharp starts it, but it is Mi Mix 1 really advertises this idea to the market.
Loose sim cards, really? I've had experience with 6 Xiaomi phones to date and never noticed that. Shaky GPS? Yes, this happened on one bottom-line Redmi, but it's hardly pervasive as far as I can tell. Software is debatable – it's not Pixel experience, I'll concede, but also definitely not TouchWiz. Battery is on average better than for most Chinese brands or for Apple, though inferior to, say, LG and Samsung. (One actual issue: camera performance is varying, typically it's bad without customization).
I won't argue that at least some models are clearly very closely copying a major brand (for instance, MiA1 is openly compared to iPhone, Mi Note 2 has Samsung S6 design), but this does not appear to be a crucial part of their business strategy (unlike the more abstract "Chinese Apple/cheap Samsung" sales pitch), and comments like yours strike me as repating a vague stereotype. In the post-iPhone era, striking design originality is rare to find, and Xiaomi doesn't seem more prone to copying than most others. They jump on the bandwagon as soon as the new technology becomes available/popular, and that's about it.