Doesn't that facilitate hacking?
Hillary used a homebrew server like ours and it was hacked multiple times. Having a physical server connected to the internet doesn't guarantee no one can hack it.
Nothing is completely unhackable, if people with money want to find a way.
You can isolate the donations through a secure payment processor so they are not stored on the online server.
There are more secure (come with a full tech team), faster, more resilient server solutions with online hosting than with a physical server in Jim's bathroom (or wherever it is located). Servers can be co-located to prevent shutdown - especially if one is located in a protected jurisdiction outside the country.