I hardly assume we know everything. There are lots of unknowns. We don't really know *why* gravity works, but it does, and we have very precise formulas which predict its effects.
There have been revolutionary breakthroughs in the past, but they've always incorporated existing knowledge and observation. Einstein's relativity was considered revolutionary, but it didn't overthrow Newton. It just refined it. Most of the time Newton and Einstein get the same answer. Most of the time people just use Newton's equations because they're simpler and get the same result. But in Einstein's time we knew they didn't work for certain situations. Newton predicted the all the movements of the planets, except for Mercury, which it got just a little bit wrong. Einstein's equations didn't make the Earth spin backwards; they just explained Mercury.
Nickel and hydrogen are common substances, and humans have been playing around with them for centuries. Over all that time, nobody has ever observed nickel transmuting into copper at low temperatures. We've been observing nuclear reactions for a pretty long time too, and they've always observed the standard model of nuclear physics, including the production of the predicted radiation, subatomic particles and isotopes. There are some some things that remain unknown or unexplained, but those things don't include fusion reactions which magically don't produce radiation.
Rossi's device has all the hallmarks of a fraud (and there have been lots of free-energy, cold fusion, LENR, etc. frauds over the years.) He claims there's a nuclear reaction, but doesn't provide any sort of cogent hypothesis for why one would be occurring, and all the evidence suggests that there is no such reaction going on. He seems to want very much to convince people that he has a working device, but seems to be working very hard to conceal any information that would let someone independently confirm his results.
There are also reasons to doubt Rossi's character. For instance, his supposed engineering degree comes from a diploma mill; not an actual university (
source).
The piezonuclear thing is quite different. They're not claiming there is some sort of nuclear process that doesn't produce radiation or the expected isotopes. Quite the contrary - they're claiming they're producing exactly that. I happen to be a bit skeptical due to the lack of reproducibility, but at least they're trying to do science and subjecting their work to peer review.
