The logging, lack of privacy was already mentioned. How about another, if you discussed a new product idea for a new startup via email, or via a phone call, or private slack message or anything else while you used one of their devices they can make a claim for the idea at any point saying you used their equipment to facilitate it. Even if they don't make a claim, they could just terminate your employment over it.
Speaking of getting terminated, if you have all your personal contacts, messages, maybe some photo's and other things on the phone and they decide one day they are going to terminate you they can require you turn in the devices with no time to remove any personal information. They can (and will win even in court if you challenge it) ask you for the passcode to review everything on the phone, laptop etc. Nothing you do on those devices is private, protected (for you) or your own property.
I worked for years in charge of a department where we managed all the IT devices, we would consistently find pictures of peoples naked spouses on their work devices, find mortgage information, bank accounts, pin numbers, you name it we saw it. I can't stress it enough, if you are issued a device from a company it is not yours to use personally. I will say, usually when we found these things we politely reminded the employee to remove them. If the person was being terminated or was under investigation we would generally dispose of these items once the investigation was complete and actions were defined. That also meant generally a few people would see these things (as few as absolutely necessary) but the employee rarely ever got them.
I also had an employee who knew he was being laid off one time decide to wipe his work laptop to prevent us from seeing what he was doing. Frankly we already knew much of it, but when he wiped his machine he caused monetary damage to the company because he also wiped a number of keys (and source code) which he created but had never properly handed off to his managers (or checked in) and it cost the company months of time and significant sums of money, not to mention lost productivity, to find and resolve all the issues. Frankly he could have been charged with a crime and/or sued for damages if the founders would've pursued him.
*edit: sentence fixed
