Let's compare this light to a brand name top end light like from light and motion -
Amazon.com : Light and Motion Urban 800 Anchor Steam Light : Sports & Outdoors
Most likely:
1. The cheaper light usually puts out about 50% of the light the brand name light does despite similar lumen ratings (cheap lights list the theoretical maximum, brand name lights list the actual output)
2. The cheaper light is far less likely to actually be waterproof
3. The cheaper light is far more likely to come with a substandard battery that doesn't last nearly as long as claimed, and will wear out sooner
4. The cheaper light is likely to use a led with a color temperature that's more purplish and harder on the eyes
5. No warantee (though let's be fair, a lot of times warranties on brand name lights are not phenomenal either, requiring proof of purchase and sending the light in and being without a light for weeks, and often paying the shipping yourself)
On the other hand:
1. You could literally buy 3 of the cheaper lights for 1 of the brand name lights because of the cost difference