My first thought on the headline are specialized concerns of the above: environment variables are an attack surface. If you use them for configuration, it's all too easy for an attacker to modify them without the victim knowing. Just look at issues with LD_PRELOAD: https://attack.mitre.org/techniques/T1574/006/
That said, I agree with GP that environment variables are super useful and super simple. But I've also been burned more than a couple of times by setting something in the past and then having it caused unexpected bugs that are hard to trace down as they aren't in my working memory. They're a double-edged sword, to be sure.
There's actually a long list of variables that are unset when invoking sudo to prevent these kinds of attacks. Systemd will also start programs with a very minimal environment that isn't inherited from any shells. You then have to specify environment variables explicitly as part of the unit file. You can also specify environment variables in environment files.
That said, I agree with GP that environment variables are super useful and super simple. But I've also been burned more than a couple of times by setting something in the past and then having it caused unexpected bugs that are hard to trace down as they aren't in my working memory. They're a double-edged sword, to be sure.