There is no one scheme. That's the first thing to wrap your head around.
Schemes vary widely. My advice is always to read the small print on them. If you do decide to go ahead, get a solicitor with experience in shared ownership.
The biggest things to remember are, they only really work in a market where prices are increasing. If they are declining then the issues with negative equity get compounded as it can be harder to sell (you don't have the same freedom to drop the price), you can't sublet out the property and it can be harder to get remortgages/staircase.
That said, it DID work for us. But I'd argue that was because our household income increased significantly.
Our neighbour had a lot more problems.
Stuff to look out for. Do you pay rent on the other part of the property. If yes, how does it increase? (is it linked to inflation). And are there other hidden service charges. (some horror stories with this).
This is why it is so important to check the details.
I would say that they are best for people who want to live in an area for a long time (preferably those with local family links and less likely to move for this reason) and those people for who renting would be the only alternative, because it offers long term security.
You also have to think in terms of your long term future. If you are a couple going for a 2 bed house and you want 3 kids, you are going to have to sell on at some point. You really want to get a house which is going to suit you longer term in case you do get stuck.
I am not either for or against them. More a 'really understand what you are getting into' type of person.