Depending on your area or even the warehouse, the job varies wildly. I have worked in WA and CA, and WA really has it together (probably because corporate can come review them at will). CA warehouses often give you too many packages for the time or send you too far away to complete in time. You also should always check your route map, because they will try to tack on extra packages that are not on your route. Evening shifts will send you all over the place and are almost impossible to complete on time. Some warehouses will give you so many boxes or so many large boxes that you cannot see out of parts of your vehicle (which is dangerous and maybe illegal).
The company will try to guilt you into continuing your deliveries over time, but I have yet to be paid for working extra. Returning packages, inability to deliver, etc. can cause you to get deactivated, which is a polite way of saying "fired, but you can appeal." You appear to be deactivated based on algorithms, so a robot will fire you, and you will have to wait weeks for someone to review your case. You may even have to escalate it to corporate.
Rules vary between the warehouses and change a lot. There are reports of warehouse workers attempting to give you bigger lots of packages based on your car size, not your route time. So, the company will attempt (and probably succeeed, because you will fear loss of income) to get free labor off you.
You will not be reimbursed mileage, parking, tolls, or any costs incurred. If packages are stolen or reported missing by a customer, it's on you. Too much responsibility is put on you, yet you have no control.
The pay is higher than pretty much any other gig economy job, but just like the rest of them, will fail in the end and hurt a lot of people struggling to get by when it does.
