Well, the main benefit is that the 25% (or so) gets pegged to house price inflation. But that still leaves the remaining 75% also inflating, so it only gets them insubstantially closer to being able to afford a full place (leastways, unless they're cashing in a London price and moving to the sticks or their income changes dramatically). I looked for some friends of ours and couldn't find any good stats on 'staircasing' (as it's known) which troubles me, because they'd be easy to pull together, so if they're not doing so there's probably a good reason. (Either that or my Google-fu is (still) lacking.)
I can understand why people do it (it's the only option available and at least you own something at the end of the day), but it's a cheap gloss on the messed-up nature of our housing market, but like most interventions, exists primarily to benefit the developers and landlords.
Check carefully the rules for selling, it's never as straightforward as selling your own house, and often more costly – the shared owner is a junior partner and gets to jump through whatever hoops are deemed reasonable (they may call it 'shared ownership', but they're still a tenant, it's 'shares' in the property they're buying). Check also the maintenance fees and liabilities, these can make up a substantial part of the monthly fees (more so if they find out the cladding is a tad on the flammable side). Maintenance costs will also often cover any surrounding grounds, many shared ownership developments are private, so residents will be paying for everything. Some of the developments I've seen in London don't seem to be ageing well. Admittedly that's true for full ownership, but the shared owner may find themselves on the hook for the entire costs.