I mean, what is there that could fail? It's basically a hole in the ground, That's it.
I know Germany prefers to strip-mine their coal these days, but digging tunnels several kilometers deep is a skill that's been mastered for more than a century.
Earthquakes, ground water, flooding, other geological activity. This stuff will be radioactive for longer than Humans will remember it is there.
These discussions are way more complicated than you apparently think, and they are both scientific and political. Nobody wants this stuff in their backyard, even a few kilometres underground. And Germany is relatively densely populated, making "nobody's backyard" a tight constraint...
I know Germany prefers to strip-mine their coal these days, but digging tunnels several kilometers deep is a skill that's been mastered for more than a century.