Some say mdma without a guide is still good but you never know . Rick doblin once got a call from a friend who was treating a woman who had depression with mdma and what happened was that she remembered her rape and her being almost murdered ,this memory was complelty blocked by her brain and mdma brought them back . More mdma did not work so Rick doblin decided lsd was better equipped to treat her which worked .
There is a video of her speaking about what happened many years later
They can all still be good without a guide, just like once you've learned to dive you do it on your own.
I think a huge part of having a guide the first few times is just so you experience it all being OK, so that if on a solo jaunt it's starting to not be OK you've got some understanding that it'll pass as, physiologically, so does the substance.
There is a video of her speaking about what happened many years later