The main flaw of the design is its basic principle: it is a constitutive system. Whatever criteria you put in or however you organise it, the practical consequences can be dire under a constitutive system.
Another key aspect that is highly problematic is that the scheme will, in the end, give you an electronic code, not a physical document. That is highly problematic in practice. People need a physical document. For other immigration statuses, people get physical documents. If people do not have that physical document, private actors will ignore them. Again, the vulnerable will struggle if they only have a code. A private landlord might not make the effort to make use of that code, or might not know how.
There are also huge IT risks. Every IT specialist we have spoken to says it is an incredible risk. Data might get lost. That happens. The data system might be hacked, which will mean that the status of these people will be gone, and they will have to reapply. If that happens, they will not have any document and will be immediately illegal, with all the consequences of the hostile environment hitting them immediately. They need a physical document. That is a key ask, because those with all other immigration statuses get a physical document. Why do these 3 million people not? It is very important.