As others have pointed out, most issues are political, not technical. There are already places which would be perfect(deep mine shafts surrounded by salt deposits, so they are literally impenetrable as salt is self-sealing) but the countries/states they are in object to keeping the waste as some sort of ideological issue.
What I want to know is whether there is a technical issue with what I am suggesting.
You just literally proposed to dig a 5-6m deep hole and assumed that would be enough...not even salt mines are enough. See Germany where there is suddenly groundwater seeking into a salt mine full of radioactive waste. A salt mine that was considered safe once. Now they are looking for years now to find a new safe spot and Germany stopped producing new waste. Still they have those problems. Tax payers are paying for this search. They'll also pay for the retrieval and they are paying for pumping out of the water.
There are many issues there obviously. This is not some easy thing you try to dig in there. It'll be there for a very long time.
My argument was also not aimed at the depth but at the unreflected "just dig a hole and hide it there" idea which reminds me of the dumping of nuclear waste into the sea.
Well, the clear difference is that stuff dumped into the sea will make it to the surface sooner or later. Containers will eventually leak and the dangerous materials will get into water, which then will carry it all over the world. Waste dumped into a 6km deep borehole is not coming out ever. I've made another comment above with a link to the wiki article about project deep borehole - where exactly this was researched and found viable for long-term storage of radioactive material. But was of course stopped by......public protests.
But at least we know were it was or came from and that pretty soon. Hiding the stuff in a deep hole and probably be forgotten before it becomes safe is just irresponsible. I would even prefer that you keep it in one of the dumps on the surface in case we do come up with an economical way to reprocess or "zapp" it in 10, 50, 100 years. At least we know where it is.
I won't even touch geological issues that may come up. A few decades ago we wouldn't know that we would be able to cause earthquakes because we're shooting chemicals into the ground. Who knows what will be in 100 years or in 200?
What I want to know is whether there is a technical issue with what I am suggesting.