I’ve had people tell me for example “This kind of herbal tea is really good for you” and then done a little research. And there’s never been any science backing it up.
Since you are specifically asking for evidence- green tea has plenty. Here are a few meta-analyses for you (though they just scratch the surface):
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8614888/
https://pubmed.ncbi.nlm.nih.gov/32372444/ (obesity)
https://www.nature.com/articles/srep06251 (blood pressure)
https://nutritionj.biomedcentral.com/articles/10.1186/s12937-020-00557-5 (blood lipids)
https://nutritionandmetabolism.biomedcentral.com/articles/10.1186/s12986-020-00469-5 (glycemic control)
All true tea (from Camellia Sinensis) has proven health benefits. Whether it’s from the polyphenols, presence of l-theanine, and more. This includes white, green, oolong, and black. Tisanes, or herbal teas, have varying health properties due to the type of herb or flower used in the brew.
Depends what you’re looking to achieve. Green tea/Match for brain health and neurprotection. Peppermint for digestion. Hibiscus for blood pressure. Ginger for nausea. Valerian, chamomile and passionflower for sleep and anxiety.
It would be a mistake to conflate not having seen studies on something with it not working or being effective. These ingredients have been used for thousands of years to serve their purpose and are often what pharmaceuticals synthesis their drugs to replicate, albeit with a stronger effect which comes with more side effects.