My MiL was a reception class teacher and a new headmistress introduced it in the school, my MiL hated it.
Qhite a number of the children in her school came from a an area of housing with a high level of poverty and deprivation. Children she described as 'talked at, talked over and talked through, but rarely talked to'. They started school with a limited vocabulary, unfamiliarity with books or pens and pencils.
She said ITA was fine for the bright children and those that were virtually reading already, they quickly learnt how to read both ITA and normal script, but she said for those children without an enriched home background and the less able, it probably set them back years and could have left them illiterate because it would be, at least the second term before they were ready to start reading, and they had barely started to grasp ITA when they moved on and then had to make the transition.
It also meant that when learning to read they could not do the random reading and recognition of words in the environment round them. I can remember my DC going through a session of reading road signs as we drove along, in the same way number recognition came on fast because they would walk round the supermarket reading prices off the shelves.
Thankfully after only a couple of years that headmistress moved on and the new headmistress didn't like ITA and everything went back to normal.