generally speaking, the process of amplification to DHT is what appears to cause the DHT like effects on things like hairline, body hair growth, voice changes, prostate volume. DHT like substances which cannot amplify via 5ar and are modified in such a way that reduces their androgenicity carry a lesser impact on prostate etc.
boldenone cannot amplify via 5ar in the human body (or at least to such low levels it is insignificant), its structural changes mean it is lessened in androgenicity, so it is more prostate friendly on a mg for mg basis than testosterone or DHT on paper.
bear in mind tho, that something that is half as androgenic, taken at multiple factors higher, is likely to still cause androgenic issues and you should assume that all AAS taken in abusive doses are not prostate or hairline friendly.