What are the risks of GnRHa blockers?
The risks aren't of taking blockers, the risks are the effects of not having hormones. Hormones are essential for many aspects of health. To leave someone without hormones for a prolonged period will cause issues.
For comparison, if a cisgender child has not started puberty by the age of 14, we start to worry. There's no immediate hurry, but just remember that hormones are needed.