i don't know about ARMs but I worked at a WeWork for a bit next to a mortgage lender and they were talking people into getting mortgages against their 401(k)s...
In my country (New Zealand), you can withdraw all except NZ$1000 from your retirement savings (usually only available at 65), if you are a first home buyer, to help you get into the market.
In the US you can borrow against something like $50k of it, but you do need a payment plan (with interest) IIRC. And you have account minimums and its only for certain purposes... etc. Thankfully our 401ks are safe from this for now.
You can imagine how that went.