One should of course never try to use personal anectodes and testimonials as a source to learn from.
Directly from drinking tea, I used to not feel anything from drinking tea but since the last half year or so I've gotten more sensitive to the effects of the tea (the caffeine really) it makes me feel anxious and nervous and my stomach hurts.
Anyway you ask specifically about long term use. There is only one thing I can think of here and that is what it does to the teeth because it contains so much fluoride. Where I live the cities do not fluoridate the water, this is rarely a problem since almost all toothpaste these days contains fluoride.
Anyway, when I was younger like until I was 18, I had pretty bad mouth hygiene because I was very depressed, I didn't often brush my teeth, plus I have always had a dry mouth from all the medicines I take, but I did drink tea every day, all the time, and I think the fluoride in the tea protected my teeth, because I never had a tooth problem until just recently at age 27 when I got a cavity, and that was only after I stopped drinking tea for half a year for the reason I described up in the top.
This despite that I have had a good normal dental hygiene for many years now.
Also all the other kids when I was in school had had dental cavities, and none of them drank tea, it could be a coincidence.
But then I didn't eat candies or drink sweet drinks unlike the other kids (i never much liked sweet things) so that could be an explanation too.
But if you live in a country where most public water sources all fluoridated like in the US, then this should not matter that much because you get plenty of fluoride just from drinking the tap water, but I guess if you are constantly sipping tea then you're more likely to have some fluoride in your mouth. However if you put sugar in your tea then that completely removes the protective effect, and is just harmful to the teeth instead, just like anything with sugar.