The only way anti-androgens alone, before initiating real HRT, can be harmful, is if you stay that way for years and years. Then, having no significant dose of either sex hormone, you'd start having issues.
But in the short and middle term, no issue.
It can be a huge help. Once you're freed from the overwhelming pressure of how your body's going to worsen (and yes, in a matter of just a year, things can get surprisingly worse - my Adam's apple appeared between 19 and 20, just as I was seeing the psychiatrist waiting for him to refer me to an endo), of the despairing libido, of the depressing aggressivity, you can engage in psychotherapy and pre-transition assessment effectively, with another idea in mind than telling them what's most advantageous so you can stop the damage ASAP.
Actually, I think everyone who's willing to transition should take anti-androgens after no more than 2-3 therapy sessions. It's just the right thing to do.
And deprived from testosterone, most real men will not feel very good. It's not absolute, but it's an additional marker/signal.