With HTB, the government isn’t lending money out of the goodness of their hearts: you don’t just repay the original loan, you also have to repay on top the same percentage by which your home has increased in value. E.g. original price was £400k and you borrowed £100k through HTB; if when you sell or remortgage the value has increased to £500k, you need to repay £125k, plus fees.
Shared ownership is developed by housing associations who can generally cross-subsidise their development and price accordingly. Whatever their faults, housing associations generally remain invested in the communities they create, and have an ongoing interest in their assets.
Help to Buy flats are built by private developers out to make as much money as possible. They build, they sell, and they leave. Prices are usually vastly inflated as a direct result of the government support, and once the development is finished the developers don’t care about the community or whether it has the necessary facilities and amenities. They’re out.
Both have a place in the market, to enable people who would otherwise be completely unable to afford to own to get a foot on the ladder, provided buyers do their research and consider whether it’s the best option for them both immediately and long term. The owners who are unhappy are generally those who saw either SO or HTB as a way of getting something lovely and shiny and new in a better location than they’d afford on the open market, and are later dissatisfied with their decision.