Cairo shoulder sits on me - he obeys me without hesitation when on my shoulder and does not bite me (except for when he is aggressive with my partner and I intervene). But I am strict - the moment Cairo opens his beak near my face (even if it is to nibble or preen), I remove him, so he has learned not to open his beak near my face.
Cairo, however, does not have shoulder rights with my partner. He has bitten my partner's face without warning, cutting him open. He does not readily step off his shoulder when my partner asks, often biting my partner's hands. We avoid giving him opportunity to reach my partner's shoulder, and I have to do a lot of body language communication to prevent escalation the situation and safely remove Cairo.
Other birds I know displacement bite and often displacement bite when on a shoulder. Majority of birds are not bite pressure trained either. I have seen the damage a beak does to a face - saw one photo of a lady whose eyelid was torn almost completely off by her ekkie. All it takes is for something in the vicinity to upset the bird and they lash out at the nearest object - you. So yes, I do 100% agree that certain birds should not be shoulder birds, especially for certain people.
Birdman666 (resident bird expert who worked with many members on training their birds and working through behavioural issues) explicitly wrote:
But she's bite pressure trained and does not displacement bite. NO BIRD should be a shoulder bird unless these prerequisites are met!
So do be sure to train that in a variety of situations before pushing the shoulder training.
Now, my feeling is - give him time. If he doesn't feel comfortable on shoulders, why force him? Just so you can walk around with him on your shoulder? A bird forced into an uncomfortable situation will often react in ways that humans do not predict.
And 9 months is not a lot of time for a parrot. It took almost 18 months for our ekkie to be ok with standing on my lap for 10 seconds. Just 10 seconds on lap standing, and it took 18 months for him to trust enough.
Sure, right now you can do a lot more with your bird than you could 9 months ago. But give it more time. Don't force it. And accept that some things some people are just uncomfortable with. You wouldn't force someone who dislikes rollercoasters to go on a rollercoaster - why force a parrot to do something that doesn't benefit them in an obvious way (to them)?