Which monthly heroes do you think were not good? Boreas is still a bit unknown. All other previous heroes were good. It's also interesting to note that from all the heroes mentioned, plus the most recent ones, zem is the worst arena hero at max level, close to useless.
Well, I never said they still are great, but Zem was the greatest back in the day (not even that long ago xD). With an awakening he could come back again as one of the best arena heroes. Tashi is still good, although recent heroes, that specialises in kill you with one shot, are making him not that usable anymore (Hurok usually targets him first). Kong can still be used vs some heroes, but not shining anymore. What I meant with those heroes was if they were really great the month they were released in. Radulf was OK, but not that usable, Jasper is OK, but not that usable, although I really like him, Valan is OK, but without skin he is only for attack, bomber squad is great, but again, with the new heroes they die SO FAST now, not even let them explode. All new heroes have made every old and not that old hero useless in arena, and they only had a place there to begin with.
The biggest fault in this was Hurok entrance. He just makes unusable most of the heroes, and now you see in arena the same pattern of heroes, not even fun. He single-handedly remove from arena 90-95% of the heroes. I had fun using non standard teams, and I am still using, but just pissed that some trashy teams just because there is Hurok can win most of the times because he spams his ultimate so much.
In competitive servers, there is no monthly hero there, because they are not that good, because they get One-hit KO from Vulko/Magnus/Hurok/Sarya, etc.. Boreas is not that good, a guy from my server has him maxed out for a while, and he really does not do much. I guess he can be good vs a 5 frontliners team, but again, Vulko/Hurok combo will just kill him in seconds.
Everything now revolves around paid heroes, free heroes are not used in main arena anymore (unless with some skill skin, not counting Malrath xD he was a cs hero as well, now free), and only used in certain teams in epic.