As you are asking for downsides I would say resentment from your other nephews and nieces and possibly your kids e.g. the ones that got their heads down and worked hard not offered the same help. Are they all "home owners" as you say or will they be paying their mortgages for 30+ years?
I think you would be naive to think your kids could inherit this house and just happily let your niece live in it until she dies. Most people inheriting a house would like to cash it in to upsize their own home, put in their pensions, reduce their hours at work etc. If you want your niece to have the right to live in the house for life you will need to get it wrapped it up legally.
Other things that could go wrong are your niece not looking after the property (is the black mould definitely not caused by lifestyle issues of not heating and ventilating adequately?), your niece not paying the rent (even if that is not passing on what is given to her by housing benefit or not claiming it) and excessive repairs that you were not anticipating.
Let's imagine your windfall is 180k plus. You could give your own kids and each niece or nephew 20k. The ones that are "home owners" could be given it to pay the sum off their mortgages (or perhaps for home improvements if you agree) the ones that are not yet homeowners you could keep it to provide a deposit when they are in a position to buy.
It's often discussed on here whether kids (in your case nieces/nephews) should be treated the same or according to their needs but in my opinion giving one a free house and the others sweet FA is a whole lot of steps too far.
If you must by the house for your niece to live in I think when you die it's value should be split between all nieces and nephews (assuming you are happy with what your kids are getting) or simply form part of your kids inheritance. Hopefully before that time your niece will have decades to improve her financial position.
It's possible in a couple of years time your niece has a great job and one of her siblings (God forbid) has a disability due to an accident or degenerative condition. Circumstances change. Your niece is also more likely to sit back and do nothing much with her life if she is being given a house because she is more needy than the rest.