My daughter has done this twice. The first time she part owned a flat which was newly built. She loved it and didn't mind the shared ownership at all but when the housing association couldn't sell a lot of the others, they rented them out to problem families and there were more difficulties. They also weren't very good at keeping the lift working so she had to 5 floors to get up and down on which wasn't idea with her disability. However, the worst bit was when she came to sell. The Housing Association determined the price which was much less than the market price and they vetoed a number of buyers which meant it was on the market for a long time. She had had to vacate the property because of her disability but they wouldn't let her rent it out, even though she was happy to allow them to accept all of the rent apart from the mortgage so she was unemployed (due to her disability) and unable to pay for 2 properties. Eventually the Housing Association had to allow her to sell the property outright but still would not allow her to sell it at the market price to recoup her losses on her bit and to get more into their purse for new housing. It was absolutely crazy and she said she would never part own again.
However, due to the new rules about the benefit system for the disabled, she decided she had no choice but to use the equity in her house to part buy a share in a partnership. She could then get the rent paid on Benefits. Any interest payments on a mortgage were repayable in full and would incur interest payments which would mean she was paying interest on interest. This a much better scheme but there are areas where it falls done. It was a new build and she needs support from the HA for any guarantee claims. The front door doesn't fit and lets in cold air (not eco friendly) but the HA aren't bothered. She wanted to move the shed, but she wasn't allowed to and many other small irritating complaints. However, it was needs must so there you go.
I'd buy outright if you can, but if you can't, it's a way of earning at least some equity in order to do so in the future.