It definitely is. I have regularly seen work reverted mistakenly during a merge. There's little fundamental difference between resolving a conflict at rebase or merge, other than one resolves the conflict you would have had with your commit when you made it, and one resolves it with the final commit in the chain. You still have to consider that the right answer is often some mix of each side of the conflict.

Not nearly as badly.

A badly done merge can indeed ruin code. But you'll always have the versions that went into the merge, and the merge itself. Your history has all of the information to recreate exactly what happened, find what changed, and then figure out how to fix it.

A badly done rebase not only ruins your work, it also removes from the branch any record of your work having been done. Unless you can find the right stray old commit which is not yet cleaned up, there is no choice but to start doing it again from scratch.