As an web dev "stuck" temporarily on Windows (the Why is a different story) I can tell you is bad because the lack of support, not being used by the majority is not tested on this platform. I spent entire days trying to fix errors on open source projects (NPM, Node, and packages) including workaround the Docker features that don't work on Windows.
I can pinpoint some examples on Why is bad:
even if you use bash for windows it has limitations, so you end up makeing .bat files for your project, duplicating some functionalities
you will write more code just for windows env (my example was Docker issues)
you will lose time on problems that the other team members do not have, so you are not efficient, basically you are wasting your employee money
if you are not careful you may put your company and team at risk, from a security point of view (it's easier to bypass or not use security features on Windows and being a more mainstream OS is more targeted)
not having the same env as your peer devs it may lead (depending on how competent everyone is) to some problems (the same as not using the same IDE), Stupid example, but real one: commits with windows line endings instead lf UNIX
some low level libraries are not compatible with Win, or they are outdated, or they behave differently, this adds more issues in the product pipeline or time to setup the Dev Machines
All of these issues may be mitigated, if you know what are you doing they will not exists, but from experience I can say this things happens.
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As an web dev "stuck" temporarily on Windows (the Why is a different story) I can tell you is bad because the lack of support, not being used by the majority is not tested on this platform. I spent entire days trying to fix errors on open source projects (NPM, Node, and packages) including workaround the Docker features that don't work on Windows.
I can pinpoint some examples on Why is bad:
All of these issues may be mitigated, if you know what are you doing they will not exists, but from experience I can say this things happens.