If I could trust my employer to not snoop around in my phone and online activities, I wouldn't have an issue having a single device for both work and private...but with their need for control and surveillance, there is no trust at all.
Never mix work and personal in a device. Anytjing that happens with work, your device may be considered evidentiary...goodbye device until that's resolved.
In my previous 2 jobs I used my own phone with dual sim, and was circumventing all this bullshit and just synching my mails from the company webserver. The method was less than official, but many IT guys and top managers did this.
My current company is much more strict, so I just carry 2 phones with me, I really hate it, but I will never give the full access to my personal device.