I was bitten by a similar problem one of the very first versions of the FB app, and I haven't tried it again. It synced my Google Contacts (what I use as my canonical contacts list) with Facebook, resulting in duplicates and a whole bunch of mess that still isn't completely resolved years later. Now that I'm on Marshmallow, I might risk it, but I'm very hesitant to do so.

> Not to mention that Facebook has intentionally changed the email address of contacts in people's phones without asking them

They changed it on FB, not in people’s phones.

That's not true, the Facebook app was "syncing" local contacts with Facebook, and thus modifying them.

Oh I wasn’t aware of this "syncing" feature; I thought it had a read-only access to your contact, that’s all. Thanks.

That's not what the article says. They changed people's own listed emails on Facebook.com, no indication they changed anything on devices.