Well, the AIU does have huge capacity. 10 accessories plus 10 switches. (a plus) I don't own an ASC or ASC2, but based on their manuals, I believe their capacity is far less.
The AIU also has those tiny screw terminals you need to insert the wires into, due to that huge capacity. (bit of a minus)
Having a DCS setup with multiple TIUs in my collection for "backup" or the "someday layout", I felt it was a good idea to buy a couple more AIUs while they were available. I'm sure the ASC units are great operationally, but I've already got some old SC-1's and an SC-2 or two that I can use with TMCC/Legacy (and Lionel of course has not announced ceasing operations in June of 2021 as MTH has, so I presume ASC units will probably be available for a while if I feel the need to add a few to my stash... )
-Dave