Sag is a big problem with tensioned leather saddles. I believe this critical design flaw, along with the invention of superior materials, is what led to their general replacement by the modern "plastic" saddle.

A Brooks saddle starts out uncomfortably hard; by the time it starts "breaking-in" it already has started to sag, too. If you ride a Brooks long enough, it will eventually sag down to the rails, despite the provision for retensioning the leather with the adjusting bolt. "Plastic" saddles will sag too as the foam padding compresses, but not as far as tensioned leather saddles.

Once a saddle sags, it becomes uncomfortable to many bicyclists, because the saddle depression prevents one from shifting their weight forward or rearward to temporary relieve a pressure hotspot. You will gravitate to the low spot despite repeated attempts to reposition. The combination of pressure, friction and road shock concentrated in one spot eventually causes rash/saddle sore/general pain in the perineum (not arse).

You don't actually sit on your arse ("sit bones") on most bikes/saddles, except in the case of extremely upright bikes like English roadsters (aka 3 speeds), which is exactly the bike for which a Brooks saddle was designed. The generous proportions of a Brooks saddle are to support more of your anatomy, the arse (sit bones). On most drop handlebar bikes, the rider's weight is supported more by the perineum than the arse. This is why skinny saddles are often as comfortable as the much larger Brooks for many bicyclists - you don't actually use/need all the saddle width of the Brooks. In fact, the wide part of a Brooks saddle actually interferes with your effort to move rearwards, so I think one could argue a Brooks is less comfortable by design. If you are a bicyclist who is unfazed by sag and never needs to reposition, then a Brooks would work well for you. For the rest of us, a modern design skinny saddle is as good or better than a B17. I'm partial to the Selle Italia Flite myself. I see they call it a "Flite 1990" now, to distinguish from the "improved" variants of the Flite made since. Someday I'd like to try a Fizik saddle.