Waste isn't a significant issue for either of the ycombinator projects. One is mostly-aneutronic fusion, the other is a fast reactor. Fast reactors produce about 1% as much waste and it goes back to the radioactivity of the original ore in a couple centuries.
Fusion has no significant risk. With fission it depends on the design; I don't know much about UPower, but the IFR, another small fast reactor, demonstrated very impressive safety.
The UPower design is waste negative so it can convert the entire planet's spent nuclear fuel and depleted uranium stockpiles into enough energy to power the globe for about 500 years. All while leaving behind a waste stream that decays to be less radioactive than the ground beneath your feet in a few hundred years. If we buried it in Paul Revere's basement when it was built, people could see it and touch it today without any exposure above background. Not to mention it is also fuel agnostic so it can run on thorium as well.
It's also important to highlight that the UPower design can consume the entire actinide vector because it uses fast neutrons. A lot of the longer lived actinides cannot be fissioned or transmuted effectively by thermal neutrons so they just build up.
We like to say we are the ultimate disposal, and can take anything, including the waste from other waste consumers.
Kudos to Sam for putting not just his money, but his time where his mouth is. If only the rest of the internet could do the same!