Playing devil's advocate... PBs were used in a far less controlled way, both in the NHS and privately, prior to the ban and many opportunities to collect data were missed. There is almost no data on the benefits/ harms from this period, and lots of anecdotal stories with strong views on both sides.
This trial will only be offered to a v small number of severely dysphoric kids in NHS clinics and only after thorough screening and MH assessments which is a million times better than what was happening before the ban. Data collection and follow up (hopefully) will be meticulous.
Hormone blockers have been used widely in adults and in smaller numbers to children with precocious puberty, and off label in gender dysphoria enough to have a reasonable understanding of the potential risks. A robust clinical trial is what's needed to determine if these drugs have any place at all in managing severe gender dysphoria in adolescents.