Over the last several years, Consumer Reports has been conducting more stringent safety testing beyond that required by the government, to be able to rank the seats on the market as Good, Better, Best. They are just now conducting testing on combination seats (seats that go from a forward facing harness to a booster seat) and discovered that several very popular brands of seats had MAJOR structural failures near the top of their weight range when used in harness mode. There haven't been any official recalls yet, but I know that many people here have mentioned owning these seats:

Britax Frontier ClickTight Harness-2-Booster
Britax Pioneer Harness-2-Booster
Cosco Finale DX
Harmony Defender 360

Here is the actual article from consumer reports:
https://www.consumerreports.org/toddler-booster-seats/child-car-seats-from-britax-cosco-harmony-break-in-consumer-reports-tests/

And a recap and some recommendations:
https://carseatblog.com/46164/consumer-reports-crash-test-findings-britax-frontier-and-pioneer-cosco-finale-harmony-defender-receive-basic-rating/?fbclid=IwAR0vF8_M4uMCMziGzlSbXXkWJpnpRV5eobFeBs0QrNuBqYmppE2CjyFreuQ