Jon, congrats on looking at multifamily. I've used the HELOC on my primary residence to purchase a 12 unit about 3 years ago, and a 33 unit in January 2019. I believe it's a good strategy, but only for the right property. I'm pretty conservative so I wanted to make sure the deal was in a good location with a value add component.
On the 12 unit, I partnered with my son. We were also doing a flip at the time, and when we completed / sold the flip, the profits covered about 1/2 the amount I'd used as a down payment for the 12 unit.
The 33 unit I purchased on my own. I've been able to significantly increase rents as units have turned over, and also added a RUBS to help cover the water / sewer / gas costs (electric is separately metered). Starting to look at cash out refi options to pay off the HELOC and get ready for the next purchase.
My thought on using a HELOC are to make sure you have a plan to pay it back. If you can't replenish your funds that way, you're going to run out of money at some point.
Also, keep in mind that by using a HELOC you're essentially doing 100% financing on the rental property. There's always a cost to the funds you're putting into a property, you're either going to pay interest on your HELOC, or you're going to have to take the money out of another investment. You've got to make sure you're satisfied with the ROI.