Before V1, you stop for an engine failure - that means for more than one in my book as well (isn't that a bit pedantic?).
An uncommanded thrust reverser deployment is a severe systems malfunction, which I would brief and stop for too before V1.
A gear collapse?? Do I have a choice to continue?
A structural failure (that is not due to a collision which I did consider above), can I have an example of that happening in the past (Wright brothers, Santos-Dumont etc. not included)?
Or did you mean after V1? In which case they are all a given too, but not really relevant to the topic. Anyway, two engines out on a twin you are not going anywhere for sure. Unlocked reverser (737) between V1 and VR you better hope for a false indication since you are sure to immediately loose control (VMCG). Collapsed gear, and structural failure - see before V1.
In my opinion the phrase "unsafe or unable to fly" was written by Boeings lawyers, not pilots.