Building the browser in allows for tighter integration, which I imagine is particularly important for an OS that's as constrained as ChromeOS (there are no native apps on the host that can just read/write to whatever files - it's not a typical Linux distro).
So the upside is a super constrained host, the downside is that any native app needs to be built in and shipped with the system. This means Chrome updates tend to be a few days to a few weeks behind on ChromeOS, which I don't think is a crazy trade-off. The major issue is that a few weeks is usually enough time to weaponize an exploit, although weaponizing one against Chrome on ChromeOS does feel particularly difficult and probably more than a few weeks of work to deliver reliably for end-to-end exploitation.
So the upside is a super constrained host, the downside is that any native app needs to be built in and shipped with the system. This means Chrome updates tend to be a few days to a few weeks behind on ChromeOS, which I don't think is a crazy trade-off. The major issue is that a few weeks is usually enough time to weaponize an exploit, although weaponizing one against Chrome on ChromeOS does feel particularly difficult and probably more than a few weeks of work to deliver reliably for end-to-end exploitation.