How are ganache-cli and geth different in terms of their specification?

I want to setup an automated testing environment and wanted some help in deciding whether I should use ganache-cli or geth to achieve the same. I looked through various resources as well as many questions here, however I couldn't find much info in terms of how geth and ganache differ especially in terms of their working under the hood.

Also, one issue that I have often faced while testing is that the same tests that pass in a ganache-cli environment often fail when I run them on geth. This made me curious to look deeper and find how these 2 tools differ under the hood and what exactly is causing the tests to fail in geth but unfortunately I couldn't find much resources which clearly explain the technical differences between the two.

From my experience and understanding both the tools basically help in setting up a similar private blockchain environment. Would highly appreciate if someone could help me resolve this.

Thanks