One of the misconceptions about HELOCs is that they are only 2nd lien loans. That is not true. In fact, almost every bank will allow the HELOC to be in first lien position which reduces your risk of having it frozen immensely. No different than the risks of mortgages. In fact, there are now dozens of Hedge Funds lined up to buy HELOC paper from banks because they have noticed over the last 5 years to be 115 times less default than a mortgage. Especially those in first lien position.
Also, the concern about the lender terminating your credit is no different than the "Acceleration Clause" everyone signs when they get a mortgage. It's the same verbiage, but you greatly reduce your risk of this if you allow the bank to take first lien as opposed to 2nd lien. Basically, just a normal refinance.
Paying extra on your mortgage is definitely one way to get out of debt faster. However, the HELOC is an open-end line of credit that allows money to move in and out 24/7. A Mortgage is closed-end. This means you can put all your income into the simple interest HELOC and when bills are due, you can use the HELOC to pay your bills. Essentially no different than your checking account. This allows all of your money to reduce the "Average Daily Principal Balance" as opposed to just paying a little more each month.
We have to remember that a mortgage was never designed to be consumer friendly. It's a debt tool used by banks to segregate income. Ever wondered why car loans, student loans, mortgages, and personal loans are all structured as installment loans? It's because they are closed-end. Money can only go in and not out. This forces the consumer to only put some of their money towards the debt, instead of all of it thus staying in debt longer and paying more interest. But the #1 goal for banks is to get your deposits for Fractional Reserve Lending purposes! So, since they are segregating your income where are you leaving the portion you don't put towards your mortgage? IN THE BANK where they want it!
So, I don't think the HELOC strategy is for everyone but I don't know too many smart wealthy individuals that finance real estate with a mortgage. For the same purposes businesses don't usually get loans...they get lines!