They are based on 18650 batteries and if you look at some online tests results, there are some coming out of China that are appalling.
Typical values per battery should be 2000 mAH+, but the trend is now to quote figures of 4000-5000 mAH which is just a lie. Many of these are actually below 1000 mAH when tested, so you battery pack may only last half as long as it should.
On the other hand these could actually be perfectly good and just not include the market for Deben, the reseller and the VAT man.