Tapping into the equity is always great when it comes to recycling money quickly as you would a flip. You are still going to have holding costs and a heloc will just have less holding costs that you would if you were doing a personal line or hard money.
There is still a risk: You do not have the right outprice and lose money, your deal goes south due to unfound things when you bought it, then the rehab phase brings them up, and you spend more, and the market changes, which is something that can happen on a moment's notice. Other risks, the contractor walking off the job, a storm hitting and delaying the project for weeks, someone breaking into the home and stealing stuff (which happened just the other day to a client of mine), a squatter, or drunk driver driving into the house (happened a lot at my old job as a firefighter lol). That being said, anything can happen, but if you run your numbers accordingly, make sure that your contractor is trusted and do all the other stuff you'll be a little more protected than the next person flipping that does not do this type of calculation.