Having a certificate for any subdomain has implications for other sibling domains, even without a wildcard certificate.

By default, web browsers are a lot less strict about Same Origin Policy for sibling domains, which enables a lot of web-based attacks (like CSRF and cookie stealing) if your able to hijack any subdomain

I did not have the money to pay the insane amounts these greedy for-profit certificate authorities asked, so I only remember the pain of trying to setup my self-signed root certificate on my several devices/browsers, and then being unable to recover my private key because I went over the top with securing it.