My standard thing to do when presented with a new idea is to google "problems with {the thing}".
Especially when the idea sounds too good to be true.
All you heard about from the proponents is the good side. They never talk about the risks.
The risk is the #1 thing you should care about.

In this case,
Google: Google Search

A Securities-Backed Line of Credit (SBLOC) lets you borrow money using your investment portfolio as backing. The major risks include maintenance calls that force asset sales during market drops, rising variable interest rates, and transfer restrictions that trap your account with your current lender.

"The appeal and convenience of SBLOCs often overshadow the inherit risks. In a volatile market, investors’ portfolios can quickly dip below the collateral threshold set by the brokerage firm. When this happens, if borrowers are unable to pay back the loan in a matter of days or offer additional collateral to meet the threshold, brokerage firms often have the right to “call the account” and unilaterally liquidate investors’ securities—choosing which securities to sell at their own discretion. One bad day on Wall Street can gut lifelong investment portfolios and leave investors with crippling tax liability. Importantly, pursuant to the SBLOC agreement, brokerage firms are not even required to notify the SBLOC investor when the account is “called” and the firm begins to liquidate securities. In many instances, the SBLOC investor learns about the account call and the subsequent liquidation the next time they check the performance of their brokerage account."

Aside from all that, the first thing I would ask is is the interest rate fixed or variable?