Puberty blockers have been used for central precocious puberty for decades - one study concluded "Results from studies to date support the use of the histrelin implant as a safe, effective, and well-tolerated option for the treatment of children with CPP."
So the question isn't really how safe they are. No gender critical people have mentioned them in that context at all. There is an issue; this drug will treat it.
So, safety concerns are just a sideline. The real issue is that the very worst gender critical people think being transgender is not a desirable outcome - Helen Joyce said "every one of those people [happy or not] is basically, you know, a huge problem to a sane world." The off-brand use is also as distraction - plenty of drugs are prescribed in this way.
Cass ignored "low quality" evidence, wrongly I believe, using the technical term, which I'd argue was bad because it's impossible to conduct "high quality" - read double blind studies with these kinds of drugs.
That said, she is not so ideologically invested that she denies the reality of gender identity, is supporting a full clinical trial for however many children who require it, and supports offering a range of mental health services to transgender children. She also, however reluctantly, acknowledges that transitioning is absolutely the right thing for some people.
So, it could be worse.