As the Account & Operations Manager of BRUSH ON BLOCK®, I hear this question fairly regularly, and I am extremely confident saying, "Yes. Mineral Powder Sunscreen definitely works!"
I have been using BRUSH ON BLOCK® SPF 30 Mineral Powder Sunscreen since 2014, long before I came to work for the company. It has been pretty much the only sunscreen I have used on my face since then, and I am happy to say I have never had a sunburn when wearing it.
In fact, it has been one of my pet peeves, reading articles by dermatologists (especially those that sell sunscreen creams and lotions for the face) who say that they recommend that their patients not rely solely on a mineral powder sunscreen, but use it to "top up" their sunscreen protection. While I know very well the importance of reapplying sunscreen every 2 hours, I take issue with their claim that mineral powder sunscreen is not reliable. But it turns out that they may have a reason for saying that, which I will get to in a moment.
When you look at a tube of Brush On Block, what is one of the first things you notice? For me, it is the big "30" on the cartridge. That Broad Spectrum SPF 30 claim is not an estimate-- Brush On Block had to be tested in an FDA-regulated lab to be able to include that on our label. And when the product undergoes testing in that lab, it is not applied over another sunscreen, or as a "top up" product. It is tested on humans, as the only form of sunscreen they are wearing at the time. So to say it isn't effective enough is to say that you simply cannot trust any sunscreen label. Brush On Block, when applied correctly, gives you SPF 30 protection, and protects from both UVA and UVB rays.
Now that one caveat that MAY be why dermatologists are concerned about mineral powder alone as sunscreen. You might have caught the little hint in last sentence of the previous paragraph, "when applied correctly." In fairness, this applies to ALL sunscreens, not just powders--people tend to not apply enough of any sunscreen. There is a concern with a translucent powder, that it can be difficult to be sure you are putting enough on, so here's the trick we use. Start out by putting a "generous" amount on the back of one hand. When I say generous, I mean that you should go over your hand two or three times. Then compare that hand to the hand that doesn't have sunscreen on it. You'll notice the difference. So then apply the same way to your face. You might look at it in a mirror, to see if you have the same look as the back of your hand. (But do it quickly, the powder and the oil from your face combine rapidly, leaving you with a natural look and not the matte, powdery look you will see on your hand. That matte look won't disappear as quickly on your hand, because it doesn't have the oil glands that your face has.) If you apply that generously to your face, you will be protected.
All of this being said, when a dermatologist recommends that you add your powder sunscreen over the top of another sunscreen, we aren't going to disagree with them. When it comes to preventing skin cancer, we're all for you using every tool in your toolbox. But remember, even if you layer your Brush On Block over a cream sunscreen, you still need to reapply EVERY 2 HOURS. You do not get "double time protection" by wearing two, you just insure that you have applied enough.