Ok, not so trivial. The isotope breakdown issue might be unsolvable (unless you have nanobots to go in and scrub out the unstable isotopes with?) but I would imagine that to be quite a bit less than you get from solar incidence. Liquid helium cooling doesn't seem like it would cause information-theoretic damage, just additional cracking. Ice crystal formation is already taken care of at this point.
But liquid helium level preservation tech really does not seem likely to be needed, given how stable LN2 already gets you. The only reason to need it is if technological progression starts taking a really long time.
If you've got good enough nanobots to remove unstable isotopes you almost certainly have the tech to do full out repair. I don't know if the radiation less than what you get from solar incidence. I suspect that it is but I also suspect that in a general underground environment much more radiation will be due to one's own body than the sun.
Cracking can include information theoretic damage if it mangles up the interface at synapses badly enough. We don't actually have a good enough understanding of how the brain stores information to really make more than very rough estimates. And cracking is also a problem for the cryonics proponents who don't self-identify with a computerized instantiation of their brain.
Within the immortalist community, cryonics is the most pessimistic possible position. Consider the following superoptimistic alternative scenarios:
Cryonics -- perfusion and vitrification at LN2 temperatures under the best conditions possible -- is by far less optimistic than any of these. Of all the possible scenarios where you end up immortal, cryonics is the least optimistic. Cryonics can work even if there is no singularity or reversal tech for thousands of years into the future. It can work under the conditions of the slowest technological growth imaginable. All it assumes is that the organization (or its descendants) can survive long enough, technology doesn't go backwards (on average), and that cryopreservation of a technically sufficient nature can predate reanimation tech.
It doesn't even require the assumption that today's best possible vitrifications are good enough. See, it's entirely plausible that it's 100 years from now when they start being good enough, and 500 years later when they figure out how to reverse them. Perhaps today's population is doomed because of this. We don't know. But the fact that we don't know what exact point is good enough is sufficient to make this a worthwhile endeavor at as early of a point as possible. It doesn't require optimism -- it simply requires deliberate, rational action. The fact is that we are late for the party. In retrospect, we should have started preserving brains hundreds of years ago. Benjamin Franklin should have gone ahead and had himself immersed in alcohol.
There's a difference between having a fear and being immobilized by it. If you have a fear that cryonics won't work -- good for you! That's a perfectly rational fear. But if that fear immobilizes you and discourages you from taking action, you've lost the game. Worse than lost, you never played.
This is something of a response to Charles Platt's recent article on Cryoptimism: Part 1 Part 2