I work with them as I manage all the absence/cover in a secondary school.
IME it isn't a job that people stay long in. But I think that is because a lot of people use it as a stepping stone to teacher training.
It is quite stressful, but it pays better than TA generally and at least you get to manage the classroom instead of deferring to someone else.
I'd say behaviour is the biggest challenge in the school I work in. There is support, but sometimes there just isn't enough to go round. I think that the better you get to know the children and they you, the better it gets.
Teachers hate coming back to a messy classroom and kids try to get away with not putting things away when they think they can get away with it (ie a supply teacher). So that's one thing to consider.
I think that keeping the kids quiet and safe is the main priority. You can only work with what you have and sometimes lesson plans are a single line of text, so there is a level of winging it at times!
Schools are very stretched right now and teachers sending in cover work from home, at 7 in the morning, when they're ill, is never going to result in an amazing, detailed lesson plan!
It isn't always possible to allocate the 'right' subjects to cover supervisors as the classes need to be covered. It's the same with teachers. Sometimes the French teacher has to cover a Science lesson or whatever. That can be challenging but teachers and SLT should be understanding about what you can do in those circumstances.