MIUI

Pro: cheap

Cons: Miui "is designed to close apps. Aggressive battery and ram management is an issue on Xiaomi. Even phones with 6gb+ ram feels like trash at some point. Even after removing battery restrictions and enabling auto start many apps gets closed."
"Devices from Chinese manufacturers [...] stop third-party apps if you leave them or lock the screen. This is a violation of Google's requirements, but Chinese companies don't care." We are several people with the same problem: many settings from Miui are fake (eg. setting the battery optimisation to none). The only apps that are allowed to run on Xiaomi are its bloatware.

And Miui don't care at all about feedbacks (eg. alarm clock not working). Don't waste your time writing them.

I tried Xiaomi, but won't do it twice.

Don't recommend. I used MIUI for about 5 years as a custom ROM. It was only around the time i broke my phone and wanted to re-install the rom again. Their website has removed ALL roms for other brands. Now they only provide ROMs to their own hardware. There are also security concerns related Chinese law that makes it ILLEGAL to refuse implementing backdoors. Avoid.

Pros:

• Good looking UI
• Frequent updates (weekly beta on xiaomi.eu)
• Passes safety net
• Good user experience out of the box if you don't care about your privacy
• You can clone apps
• You can protect access to your apps with a password

Cons:

• Ads on global version of the ROM
• You have to accept their fishy privacy policy in every stock app to be able to use them
• There's built-in firewall, but you can't block internet access for system apps (those apps keep connecting to Amazon and some Xiaomi servers)

Good alternatives:

• LineageOS + Gapps (option for people who can't live without Google)
• LineageOS + microG (microG replaces proprietary google services with it's open-source implementation. Some applications need it to be able to send you push notifications)

• Modern Iphone Ui,
• Included App Lock, Dual Apps, Long Screenshot, Micloud, Network Meter, Ota Update, Second Space, Theme Manager

• Banner Ads On Every Built In Apps
• Bloatware Such As Apps, Mi Community, Etc

MIUI includes various features such as theming support.