I studied Vedanta in India at an Ashram for 4 years continuously without break.
One of the things my master would emphasise a lot was that physical yoga is the first stage on the spiritual path and meditation is the last. But he says they are easy to teach and have become completely commercialised in the West only because they are both really easy to instruct.
He cautioned us against the perils of meditation when you are unqualified, pointing out that it may lead to mental illness. In general, the qualification for meditation is total uparati, withdrawal from the world. A state of mind where the objects and beings of the external world have no sway on the mind at all — he would say “there is no push from within, there is no pull from without.” He said trying to meditate with the mind of a typical Westerner is like suddenly putting a brick wall in front of a car speeding on the highway. You will crash and it will be painful.
Some of the Upanisads have lines which repeat which say “this must not be spoken of” — being suitable only for prepared students, but now people walk around talking about illusions and maya like it’s just another How to Win Friends and Influence People or something.
If anyone is interested in reading his perspective on meditation, the chapter with the same name in Vedanta Treatise by A Parthasarathy is instructive.