Seems like a good deal. Bad news is this brand isn't very good?

Seems like a good deal. Bad news is this brand isn't very good?

Which product category do you mean when you say it's not good to use? The Advanced Night Repair and the foundation have pretty different reputations—maybe it's not an issue with the brand as a whole, but rather that a particular line doesn't suit you. Looking at it from another angle, there might still be individual products that work for you.
Is this CB cashback or a coupon?
The main risks are thresholds and timing: high cashback often comes with minimum spend requirements, restricted categories, or limited-time settlement. Before buying, it's best to confirm whether it covers the specific item you want, and whether the cashback will be clawed back if you cancel the order—don't just look at the discount figure.
Another hassle when checking out is hitting the minimum spend and dealing with taxes. Cashback rates are usually calculated on the item subtotal only, excluding shipping and taxes, so if you don't reach the threshold you end up spending more—and it's easy to get burned when you tally up the total.
The key is to first figure out whether you actually need it—otherwise, even at 40% off, you're just buying another bottle to sit unused.
Hold off on checking out just yet—40% cashback sounds tempting, but pay attention to how and when you'll actually get it. Many rebates only pay out after a settlement period, and they may be voided if you return the item. Try a sample in-store first or compare with what you already own to make sure you'll really use it before buying, so you don't end up stockpiling a bunch of unused stuff just for the discount.