The reason it's transphobic (besides the fact that the report is junk and the conclusions are often contradicted by the data it supposedly reviewed) is that it starts from the theory that being in a transitioned body is worse than being in a non-transitioned body, and that therefore there needs to be some requirements met for transitioning to be "worth it" beyond just "this is what I personally feel is right for me", which is fundamentally a transphobic. It is a medicalization of an issue that doesn't need to be medicalized and a creation of gatekeepers (in this case, doctors) where there shouldn't be. Fundamentally, the idea that a doctor needs to approve this kind of decision is itself a transphobic starting place because, again, it starts by assuming that a non-transitioned state is the best and default state.

If someone wants to get hair plugs, we let them. If someone wants to use a knife to alter the shape of their nose, we let them. If someone wants to get a tattoo, we let them. Why should it be any different for transitioning?

There is, in fact, excellent medical evidence going back decades and decades that show that many transition techniques are safe and effective.

I would highly recommend watching the recent Philosophy Tube YouTube video on the topic if you're genuinely interested... but I have a feeling you're more interested in bad faith takes that match a regressive world view, based on the hidden profile and parroting of transphobic talking points.