In our experience you really have no support as an SGO family as SS hand over the child and literally walk away.
I think this point, by flapjack, is the key one. I only have second hand knowledge, so take with a pinch of salt, but I hear bad things about SGOs being used as a cheap way to avoid adoption and not provide support.
For most other purposes an SGO will be fine for the reasons already discussed on the thread. My understanding is that wildthings point about the difficulty is correct, but not absolute.
Still, if both were equally valid in the family situation, I would personally go for adoption, because I would want the full PR and the legal title of parent. I would want to be able to ensure consistency for the child.
I don't understand why you say "hugely intrusive draconian step"
I think objectively that is true- draconian, in that it is essentially a form of force; and intrusive, in that the state is breaking a fundamental relationship. Doesn't stop it being somewhat insensitive phrasing for this board, but I'm sure wildthings was just meaning from the view of the court process.
There are lots of other adjectives you could use about adoption that would be just as true: life changing, life saving, hope giving, positive, loving, generous, brave, selfless... I could go on, but ultimately, once that court process is over, the important bit is that it's just a different way kids get their parents.