Sejs said:You, as a DM, should not be afraid one bit. You know how there's that saying "There's always somebody bigger"? You're everybody - by default, you've got access to something that will challenge the party, FB or no. It's just a matter of finding the balance between challenging the FB and at the same time not utterly obliterating the rest of the group. But hey, that's what DMing is all about anyway, so no worries.
Well, gee, that was enlightening.
Taking this argument to its logical conclusion, we find that letting a 20th level guy adventure with a 1st level one is perfectly fine, because hey, finding the balance between challenging the one guy while not obliterating the other is what DMing is all about.
The art in DMing is how one builds these challenges, and what threshold to set beyond which characters are disallowed because finding challenges for them starts becoming infeasible for whatever reason. Yes, all character concepts are possible. Not all are practical.
No, the people who should be worried are the other players. A Frenzied Berzerker is a ticking bomb. Eventually it's going to go off - and as a DM, let it. Don't pull any punches or let the player try to squirm out of* what is supposed to be one of the primary drawbacks of the PrC. If he wants to ride the fast train, he has to pay the ticket price.
You misspelled "everyone else has to pay the ticket price". Hope this helps!