Mostly, they're too bright with an unfocused beam (so may dazzle other road users, which is both illegal and dangerous), or they have a steady mode with a crap lens (a technical fail, probably not dangerous), but the helmet light will unavoidably show a red light to the front and white light to the rear if you ever turn your head (dangerous, confusing other road users about your direction, only not illegal because it's on the rider not the bike).
As discussed in a couple of other topics at the moment, you're unlikely to attract police attention unless it's painfully obvious dazzling (and I'm not sure if any are quite that bad) and if you do, it'll probably only be a £50 fine, but with legal lights being so cheap now (Lidl did a set in their recent specials for £10, while online retailers are often similar-priced but postage extra), why bother with tat, especially those with dangerous faults?