Well, they were made popular in the 1980s by the late Col Jeff Cooper (Who I knew). What many never got about the forward mounted scope on his 'Scout Rifle' was that it was there mainly so stripper clips could be used, from above. The introduction of replaceable box mags on many bolt guns since has made that unnecessary. The second reason is that some say, (and find) they can use a forward mounted scope with both eyes open, and thus get a wide field of view around the scope. Keeping in mind in Coopers day red dot sights were in their infancy, that option has also, arguably, become better served since.
In short, while I think they must work for some, I see very few who use them and the lack of range offered by the manufactures rather suggests demand is low. As a footnote I once read a book written by a German sniper in WW2. In it he complained about the forward mounted scope on the K98 and compared them very unfavourably with the rear mounted scopes on the Soviet Mosin Nagants. He also said how happy he was when the K98s were later issued with rear mounted scopes. I guess some would argue the forward mounted scope is an instinctive both eyes open short to medium range set up. But then, we now have excellent and reliable red dots for that.
In my case, the reason for considering it is for a K98. A rifle I have tried to scope for years, and failed to do satisfactorily.