Well, the objections are two-fold:
1) They tend to die far quicker than they did, historically. Several lasted into the 18th Century.
2) Why does it seem that just Hordes are being marked as non-playable and/or doomed to be conquered? Why are Sub-saharan nations and Native American nations, that are generally considered far less developed, allowed to advance/survive/thrive, if they reach the appropriate tech levels, but Hordes are not?
the exact timing is really hard to reach... it will always been a little bit soon or a little bit late, the actual hordes outcome is not bad at all...
Sub-saharan not on the coastline survived the EUIV timeline, Europe didn't care about inner africa until the XIX century, so there is nothing wrong if they survive inside that continent...
even some north american tribes can be considered to have survied until the endgame...
no hordes have done that (and we are not talking of a single country, where you could say Aragon could have survived if not annexed, we are talking about an entire nomadic lifestyle, culture and goverment form that faded away... if they didn't survive, the reason for that must have been a sound one)...
if the hordes are a military threat in the early game it's fine, if they survive 100 years less or more then in history it's fine...
but if they are still around as military threat in the endgame it will be really bad...
so i consider the actual balance of the hordes pretty good...
P.S. i'm always talking of a game only with ai and with no player intervention...