The main downsides of windows for a server are:
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Forced reboots
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More RAM/Storage usage for the OS
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No options for ZFS or similar data protection software, storage spaces provides basic RAID but the performance can be fairly low.
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Needs a license
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Less general availability of self-hosted software, but you can run Docker for Windows as a way around that.
However there are some upsides, it's very easy to set up and manage, SMB shares are super easy, and some backup software like Veeam B&R is windows only.
You'll get better performance from a 7th Gen core i5 box with about 1/10th the power usage, those go for $80 or so usually.