I'm not up with the finer details of the tech, but if the sales pitch becomes reality they could be promising.
Main issue is that I've been hearing about how gen IV reactors will revolutionaise power generation ANY DAY NOW for the last 15 years, and it still doesn't seem to be on the verge of becoming commercially viable tech at a large scale. The roadmap of the international gen IV forum targets commercial availability as 2030, but this is basically advertising copy and I expect significant overruns to that, given that major overruns ALWAYS happen when you're trying to productionise cutting-edge science in a hurry - the f35 or any other major military procurement program for example. 2030 is simply too late, and 2040 or whenever the things are REALLY ready is even worse. We need to make major inroads into emissions before then. Once it IS available, then there's no technical reason that gen IV nuclear shouldn't be a long-term part of the energy mix, but I'm not counting on it being a realistic option on the timescales we have to avoid climate tipping points.
But the issues with waste and location don't go away. Sure, gen IV reactors produce less waste and it's 'only' dangerous for centuries rather than hundreds of millennia, but that's not going to reassure the people who live near the waste dump. And a gen IV reactor is still a reactor, and I suspect there's not going to be a lot of enthusiasm for having on in your neighbourhood, so the task of building them will be politically complicated. And maybe with some reason - when they're ready, gen IV reactors will be brand new tech, and brand new tech, by its very nature, has bugs and goes wrong and there aren't many experts around to help you out with it.