What do you mean by "Safe"? Investment has a risk of loss, its possible and highly likely that in the next 10 years your 401K will loose half or near half of its value. At some point it may rebound but markets move, sometimes a lot. Now if the question is will my 401k be confiscated by the government? Then probably not, but if things get bad enough many governments force citizens to buy government bonds with their pension investments so that's always a possibility but not likely in the next 5 years.
I know investing is always a risk, so I don't mean dropping in market value or whatever. I mean more like someone saying that even though I put all that money in my 401k and it technically still exists, I don't get to have it. Employers. Banks. Government. Whoever.
In a crisis situation the rules change mid game. The current rules don't allow seizure of 401k but if a crisis happens everything is on the table from bank accounts to 401ks to gold and guns there really isn't any asset class that's "safe" from that level of risk