Caffeine has negative side effects. It is an addictive psychoactive drug. Its effects are mild, but, to repeat, it is an addictive psychoactive drug.
Even moderate use can cause sever anxiety in some people. L-theanine is a great counter to this. Green tea naturally has L-theanine in it and that is likely why people who drink green tea do not report anxiety (at least in significant numbers) while coffee drinkers do.
FWIW Matcha has even more L-theanine that green tea.
From personal experience, it caused my medications to stop working.
Now that I did not know. Given that l-theanine is a natural constituent of green tea (though I don't know offhand in what amount) I figure there would be more mention of this! A casual search doesn't reveal much, and even examine.com (https://examine.com/supplements/theanine/research/#pharmacol...) doesn't have anything listed. Ugh.
Then again almost no one mentions that 5-HTP can cause horrible things to happen to your heart. :/
It's an important distinction to make in this discussion, which largely boils down to "off label use", whether we're talking about drinking simply too much coffee or people with developing brains getting too much thc.