Hi, first of all: Thanks for your work!
I noticed the following inconsistencies between Windows and Mac/Linux:

On Mac/Linux an absolute path to an executable file returns true
e.g. /usr/bin/ssh -> true

On Windows an absolute path is not accepted:
e.g. c:\Program Files\putty\plink.exe -> false

While this just seems to be an inconsistency, the following is certainly a bug:

On Mac/Linux all directories are recognized as "existing commands":
e.g. /usr -> true