@perpetualplatespinning Thank you. I wasn't aware that ASD units were available for post 16. That's interesting. Sadly there are none in our borough.
OP - my DS is in an autism resourced unit attached to a mainstream secondary. He attends all his lessons - with adult support. He's KS4 now. When he was KS3 there were some subjects he didn't attend though (eg MFL, drama) but he was still expected to attend all core subjects.
The unit does allow him to have a place to go when he gets overwhelmed though and he can go there at lunchtime to decompress from the pressure of being around 2,000 students.
His school does have 'nurture' classes for students working below a certain level which are much smaller but they're available to all students, not just those in the unit. They're only in maths, english and science though.
You mention 'small class sizes' and 'ratio of teachers', to me that sounds more like a specialist setting than a mainstream secondary, even if your DS were to get into a unit.
I would also say that you need to consider the profile of the other students in the year group if your DS does join a unit. Units tend to be very small - maybe 4/5 students in each year group. You will need to make sure he would have a peer group.
As others have said though, every unit is different and there are some in my borough where students do spend most of their time in the unit. I would be doing some research though to find out what those DC are doing all day and who is actually teaching them. I would want to know what their outcomes and destinations are. I know it is very difficult to find staff for these units and turnover can be high.