Seems to me that she is expressing concern about something that may be occurring, while also acknowledging that many people can benefit from treatment. Basically that more research needs to be done to see who benefits and who may regret it.
Can you quote the part you think is problematic?
J.K. Rowling Writes about Her Reasons for Speaking out on Sex and Gender Issues - J.K. Rowling
The essay is quite long, and I don't think I would be able to list every single part I have a problem with and address it sufficiently in one post. But just for a few examples:
I’m concerned about the huge explosion in young women wishing to transition and also about the increasing numbers who seem to be detransitioning (returning to their original sex), because they regret taking steps that have, in some cases, altered their bodies irrevocably, and taken away their fertility.
In 2018, American physician and researcher Lisa Littman set out to explore it. In an interview, she said:
‘Parents online were describing a very unusual pattern of transgender-identification where multiple friends and even entire friend groups became transgender-identified at the same time. I would have been remiss had I not considered social contagion and peer influences as potential factors.’
Littman mentioned Tumblr, Reddit, Instagram and YouTube as contributing factors to Rapid Onset Gender Dysphoria, where she believes that in the realm of transgender identification ‘youth have created particularly insular echo chambers.’
I want to be very clear here: I know transition will be a solution for some gender dysphoric people, although I’m also aware through extensive research that studies have consistently shown that between 60-90% of gender dysphoric teens will grow out of their dysphoria.
First, available evidence shows that only a small minority of trans children "desist" or "grow out of gender dysphoria." The "60-90%" claim is overwhelmingly false and unevidenced, at least if we go by current science and not fringe voices:
Transgender kids tend to maintain their identities as they grow up, study suggests
How many transgender kids grow up to stay trans?
The Controversial Research on 'Desistance' in Transgender Youth | KQED
Second, there's no evidence that "increasing numbers" of trans women are detransitioning. The wording itself is misleading: since transitioning is now more accessible and more socially accepted than before, any given percentage of detransitioning is going to result in larger numbers now that more people are able to transition. But the percentage itself hasn't been established to be large or to have increased in the first place: 2% (just a random number I'm using as an example) of 1,000 is 20, but 2% of a million is 20,000. The same percentage amounts to a wildly different number.
This article addresses more of the claims she has made, although I haven't cross-checked it yet with many other sources. It responds in detail to some of her points, though (despite the clickbaity title):
This Is The Sequel J.K. Rowling Doesn’t Want You To Read
I'm not a woman, but I can see why some women, especially those who have been sexually assaulted, might feel intimidated by allowing transwomen into women's bathrooms, changing rooms, etc.
In my hometown, there was a women's refuge for victims of violence that was recently criticised as 'transphobic' for not allowing transwomen.
To me it seems like an issue of competing rights which can only be decided upon by a subjective value judgement about whose rights should be given precedence.
Seeing as bigoted those who in good faith prefer one over the other seems wrong to me, and also a serious roadblock against finding the best solutions.
I have commented before, including on this forum, that there are indeed valid concerns about unconditionally allowing people into all spaces for women as long as they identify as women. For instance, there's a lot of evidence that seeing male genitalia can trigger PTSD responses in some survivors of sexual assault. I also see other areas, such as sports, where there are a lot of issues that still need to be discussed, researched, and worked on.
All of this is important to talk about, as is the subject of minors undergoing surgery or hormone therapy, or children taking puberty blockers. However, I don't see an influential public figure who makes unevidenced claims on such a complicated and, to many, life-changing issue to be acting in good faith... and even if we assume she is, the practical outcome of what she's doing is largely negative. Any valid concerns she could have expressed have been drowned out by her support for unambiguously anti-trans people (such as Matt Walsh), hasty conclusions (e.g., the idea that men will intentionally identify as women to assault women in bathrooms), and the unevidenced claims (e.g., that "60-90%" of trans kids "grow out of" gender dysphoria).
There are many ways to talk about these issues and discuss them productively, and I do think that some people are too quick to react negatively to even the suggestion that, say, trans women shouldn't automatically be allowed to compete against cis women in sports. But J. K. Rowling seems to me a clear example of someone whose input on the issue has been unproductive and harmful far more often than not.