This is one of the solutions that our LA has suggested for DS1 for secondary. Our problem is that it doesn't even exist yet, so I cannot visit it to find out how it works or evaluate whether it would be a suitable option for him.
DS1 is extremely bright, but has very pronounced social and sensory difficulties. He hasn't even been in his own classroom at his small MS primary for more than short periods, this week. This is a class of under 20 children, most of whom he has known since he was 3, so as MS classes go, it's pretty undemanding.
My concern about an ASD unit is that he would never willingly venture out of it to go to lessons and would end up doing little more than worksheets all week and becoming thoroughly bored with the whole thing - and finally give up on school altogether. I can see how one would work for children who can manage in class with support, but he's not managing in class with support. He's not even in class half the time.
So, if you have an able DC with ASD in a unit or base within MS, what does their day look like, in terms of organisation? Is your DC accessing a reasonably full timetable, and do you think the support of the base is enabling this or that it would happen, anyhow? Do you think the system is a good solution for your DC, or have to had to settle for the least bad option offered? If your DC gets out to mainstream classes, how well supported are they in those lessons? If not, are they being properly taught?