I have experience of working with very good assistant practitioners and negative memories of one particular practitioner.
All those who I have worked with supported registered practitioners in a role that they had been trained to occupy by the registered nurses with whom they worked. Where this worked well, the assistant practitioners carried out - for example - the assessment of incontinence, and were able to give appropriate advice; but were also aware when they needed to discuss / refer the patient to the registered nurses. This usually worked beautifully, but if things did not run smoothly - as I mentioned above - this could have equally negative repercussions on others in the team.
It will be interesting to see what others reply to your question.
I have experience of working with assistant practitioners (AP) in Endoscopy and clinical research settings. At my new Trust I have met assistant practitioners in working with the surgical team doing all the pre-op checks. So I think accountability and delegation depends on and in area of practice.
They were all competent and in clinical research: AP were given own projects (assessed and deemed appropriate), later on 2 went on to Med School, one to train as a Physio and another as a nurse. In endoscopy APs could undertake pre-assessment, discharge, perform cannulation and venipuncture etc. They had done level 4 NVQs, trained and assessed in these roles.
So role might have to fit the purpose you require, you need a clear job description and requirements, get the right candidates, inform or educate all other staff in clinical area what the role is about for acceptance, knowledge and understanding of role. This will make it easier for delegation of roles, knowing the boundaries and expectations from all involved. They will need clear training pathway and with this you are ensuring they are adequately trained to work in role and you can facilitate supervision, appraisal and other methods of evaluation focused on this. Additionally, you will need opportunities mapped out about how role can be further developed and adequate remuneration to make it attractive. With regards to accountability, they are not covered by NMC code of professional conduct, but the professional who has delegated is. Therefore as long as they are working within the scope of role they should be covered. This is speculation as I don't know if it applies to APs...., but maybe in case of misconduct they could be judged against someone in similar role or as per Trust or department guidelines. Hope this helps.