To me, in general, compiling seems to imply native output where optimizations are done while transpiling seems to target another high-level language where semantics are kept as close to the original as possible.
I like the reasoning in the sibling comment:
> So if you want to understand what other people are saying about "compilers" you should keep in mind that the standard definition includes what you are calling "transpilers".
Really, the main disadvantage of saying "transpile" is that it implies you don't know what "compile" means. In a context where that wasn't an issue -- e.g., if I heard it from one of my coworkers, who certainly know what a compiler is -- I would just think they liked the neologism.