I'm not sure such a thing really exists.
If a low back booster fits the child properly and puts the seat belt in the right place, and if the child will sit properly, it is safe.
The problem, of course, is that low back boosters tend to NOT fit young booster riders well and younger riders tend NOT to sit properly. But that's not something that's very easy to summarize in a FB-friendly guide. There isn't anything as simple as "High back boosters are safer for kids under 60 lbs" or something because that is going to depend on the child, the vehicle, and the booster. A well-fitting low-back booster is better than a poorly-fitting high back booster.