Aside from the nature of synthetics to grow cancer cells (if that's true), there is also an assumed risk for natural progesterone not being able to keep oestrogen under control and, thus, allowing oestrogen driven cancers to proliferate.
I don't agree with this but many doctors only trust synthetics to be strong enough. Private BHRT docs are doing wonders with bespoke progesterone to do a gentler but thorough job of balancing oestrogen. I'm sure it will all trickle down to the NHS eventually.
Our own oestrogen in peri can be dangerous if it keeps spiking too high, regardless of those on HRT. AT least with HRT you can control how much oestrogen you take so more needs to be done to keep an eye on peri women not on HRT, who may be assumed as risk free.
I had to have a biopsy for many small and 2 large breast cysts earlier in the year and the first thing they asked was whether I was on oestrogen HRT as there were so many. The only option we have is the combined pill via the NHS. Shocking really.