No. A couple of settings get reset like privacy settings i.e. which all apps have been marked to ask pass before opening, nothing serious though.
Overall, it's a refreshing change, and it's much much smoother now.
Many required options are a welcome change for ex- call/sms pref for any particular SIM; earlier it asked every time you dialed a no.
Some things have been left out for ex the lock-screen earlier provided options to open messaging or dialer directly, which is now restricted to camera only. Plus, the app drawer had option to go to other toggles, now there are fixed 9 toggles, which can be just replaced with other toggles. But I normally require more than 9 which troubles me.
Overall, it's a must update. But you have to first update to v56, then only it'll show MiUI v6.6 update.
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One more thing, it unroots the phone, and I guess the older root method doesn't work for this.