High back booster seats are safer than backless booster seats because the back of the booster seat helps to protect the child sitting in the high back booster seat more than a booster seat without the backless booster seat.
But even a backless booster seat is safer than having no booster seat at all.
The back of the regular seat in the vehicle will also protect the child.
Mainly a booster car seat helps to raise the child up high enough in the seat so that the seat belt in the car fits around the child properly.
The booster seat without the high back cost more money than the backless booster seat too.
My kids used a backless booster seat which they found more comfortable instead of the high back booster seat.
Thankfully we never were in a car accident to need to test the booster seat.