I have a SO flat, as two of my friends, we've all had quite different experiences. As a note we're all in central London and we're single when we bought them.
I got an absolute bargain on the rental % of my two bed, I think it may have been priced wrong. The flat valuation was quite high. So I bought the minimum share and have very low ongoing cost. If I sell now the flat is probably worth a bit less than it was valued at 8 years ago when I bought it, so I'm sort of stuck here as I have no incentive to move as it's so cheap. Service charge keeps going up, so be aware of that, I have no dealings with the HA, which keeps changing as they acquire one another.
Another friend bought a two bed more at market price, she ended up staircasing to 100% as was doing well. Given she bought the remainder at the beginning of covid the valuation she received to buy the remainder was low, so she would have made a profit if she was able to sell.
A third friend bought one one bed that seemed a low valuation at the time and enjoyed living in it, but his total costs weren't terribly low. He's now being trying to sell for about 18 months and no one is interested. He's reduced it a few times, but the timing just isn't right and anyone buying a SO tends to want a new one. The costs to sell are also high, so even though technically the valuation has gone up, the HA costs wipe out most of that gain.
So a mixed bag really.