Integrated seats are safe ... so long (as always with any restraint system) they are used correctly: follow the manual.
The most important risk I think is that when kiddo needs to ride in an alternate vehicle -- such as during a road side emergency -- so I would at least keep that spare
booster in the trunk.
Another important factor is whether or not your child will slump out of position. When kids fall asleep in backless boosters, they usually lean out of safe position. I was taught that any highback
booster is about 70% safer than any backless.
How to best protect children in side-impact crashes
Researchers from The Children's Hospital of Philadelphia say restraints, other passengers in rear seat reduce risk