Not heard any 'horror stories' about remortgaging. We bought a Charles Church new build using 20% HTB equity @ £48k and the plan was to remortgage at the end of the 5 year fixed to pay it off.
Now though we went round to the sales office to complain about the road still not being finished with tarmac (2 years on) and ended up going in the show home so now we're buying a newer and bigger build

women. We are selling ours and paying back the Govt which means they take 20% of the sale value (which is now iro £300k so they'll take £60k) and we are allowed to apply for a new HTB equity loan so the paper work is moving forward on that now.
The HTB is good as long as you know what happens after the 5 year grace period in that you start paying off interest on it and it isn't cheap interest as it staircases each year inline with inflation, the Govt wouldn't have created the scheme if there wasn't money to be made when they increase rates inevitably over the next few years.
Also the other pitfall is that on the new paperwork you have to add who your mortgage / independent financial adviser is, we're inquiring at the moment but it appears they will only provide the funds if you go through a mortgage broker and not direct to a bank which sucks because we've got a mortgage in place with Barclays at the best rate we could find and a mortgage broker won't get anything below 2.89% I suspect and nothing close to our 2.69% offer.
Hopefully they don't require a financial advisor (though they are useful as the conduit between Bank, Govt and Solicitors and usually take their fee from the mortgage lender so you don't pay anything)
Whilst I remember, the equity loan is non-portable and must be over a 25 year period, they try make your mortgage mirror the 25 year period but it's not mandatory. What I have noticed is that most of the mortgages that go with the equity loans also have chunky early repayment charges, obviously trying to discourage re-mortgaging but to be honest unless you have a windfall of cash, the only way is to remortgage. In my eyes, that's the whole point of the bleeding thing.
I would recommend it if you don't have the equity for a decent deposit like most of us don't - though I *think* the scheme ends this December and only applications in process will be accepted and not new applications. Also they now require direct debit details for the equity loan so they can take £1 a month to ensure the account is active and it also classes as the first 60 months 'interest'.
Any questions I'm happy to try answer as someone who has and is going to re-use the scheme.