Hey @Daniel Haley It's a good question.
Pros - once set up you can basically make cash offers and close quickly on any property, up to your HELOC limit. Cash deals and quick closes can save quite a bit on the purchase price for some buyers.
Cons-It may not be the best way to finance a rental property permanently given a probably higher interest rate and higher payment.
HELOCs can work with various strategies so it really depends on the road you are planning on traveling. I am closing on a HELOC in the near future for my primary residence. This will allow me to essentially make cash offers on properties. I would generally only use the HELOC over a conventional mortgage if I could get a significant savings on the purchase price.
My current goal is to secure a flip or BRRR property where I can buy and rehab it with the HELOC then sell or rent/refinance and get my money back and find another. I have a full time job and am only looking for one property at a time at this point.