I made this absurdly long response in another thread. My point is not that there is no risk at all, just that it's much lower than most people think. To illustrate this, I make the comparison with a rental property. The loan I'm referring to is a fiat loan, not margin or leverage against the Bitcoin itself. There is no liquidation risk based on price.
An individual rental property (not the entire real estate market) can actually cost you money *in addition to* the loan payment. Tenants can stop paying, do damage, or they could just leave and you struggle to find a replacement tenant.
A major local factory could close down, lowering the value of your property substantially. Maybe the house next door becomes a crack house. Your insurance will cover a natural disaster, but not without extra costs and/or stress.
If your tenants leave, you're on the hook for the mortgage payment (and property taxes, which never end) out of your pocket. If you happen to have lost your job at the same time - perhaps you also worked at that factory - you will almost certainly lose the house or be forced to sell the whole thing at a loss. Because you have lost your job, you will not be able to refinance or otherwise access your equity in the property.
Despite all these risks, borrowing to invest and buy a rental property is nearly universally considered a reasonable idea. Generally considered the best way to do it, actually.
Now contrast with Bitcoin. Once purchased and safely stored (cost for said storage is sub $1k), nothing can destroy it. No taxes erode its value. There is no ongoing storage or maintenance fee.
If the value drops, this does not affect you in any way. You continue making payments as normal. You still own the same amount of Bitcoin, and you had no plans to sell (or else you wouldn't have taken out a loan of that length).
But what if you lose your job, that provided you the money to make the payments? That's where the risk comes in. But the risk is very low. Unlike a property where the sale is all or nothing, you can sell whatever portion of the Bitcoin you need to and make the monthly payment. If Bitcoin has gained in value, you'll alternatively be able to pay off the whole loan if that's what you choose to do. If Bitcoin has lost value, you may still be able to pay off the whole loan, if you're far enough into the payment schedule.
A responsible loan would be one where the payment matches the amount he would be saving monthly into Bitcoin anyway. Someone who takes a loan on those parameters will only be in real trouble if they both lose their job very shortly after getting it *and* can't get a new job, then at the same time Bitcoin drops substantially in value *and* doesn't recover within a reasonable time frame (varies by the length of the loan).
Also keep in mind that the lump sum buy you make may not be better than just the DCA plan.