HotI - the issue is with the positional injuries that are more likely with the harness, and that they do not perform any better in accidents in terms of preventing injury. The issue seems to stem from the issue of incorrect installation or incorrect application, which is exceptionally easy to do and difficult to get right. Another paper also suggests that a major flaw in the H Harness design is that it is attached directly to the bolt anchor point, not the seat, and this directs massive force to the neck and head.

You can read one study here:
Accessory child safety harnesses： Do the risks outweigh the benefits？ - 豆丁网
Yep what Lime said! The H harnesses are usually more trouble than they are worth and misuse is very common as they aren't the easiest to use. They will do damage if not used right. If your DD can sit properly and still for the whole car trip then the lap sash will be better than the H harness. If you're finding she has trouble sitting still and the seat belt placement isn't always staying correct, you might have to consider changing to a booster that will keep her in a harness for longer. If belt placement isn't right it won't protect her in an accident. There are quite a few out there now that can harness even 6 & 7 year olds.