I remember the ITA fiasco from DS's early school days.
He was beginning to read before he went to school, but then was completely baffled by ITA books. It put his reading back considerably, until I decided to take charge and taught him using the conventional alphabet.
He's an enthusiastic reader to this day, so it didn't harm him in the long run.
I think the main problem with the ITA was how the changeover between the two alphabets was handled. Ideally, pupils reading development would be closely monitored, and the changeover adjusted to each child's needs.
But teachers were poorly trained for this, and with large classes of forty or more pupils it was impossible to provide the close support needed for a successful transition.