Well, you are aware that you could be stuck with a rising interest rate on a HELOC. That’s the big drawback, no matter where you go. And you say you won’t use it till you need it — but your “need” will likely arise in a period when rates could be rising! Also, yu know that unless you use the proceeds to “improve” your home, the interest will not be tax deductible.
So check not only the current initial rate (which you are not using) but also the formula for future rate increases, the cap on rate increases, and any drawdown fees before making your decision. And a credit union is as good as a bank. They’re pretty easy to join these days, but most will require a deposit of some sort in an account to become a member.