I don't think it's a good idea for newbies to try to 1v1 taira, since taira has mountains and a quite good army. It might be better to have them just skip the fujiwara PU.
In addition, just in terms of japan uniting strategies, I like going PU taira -> PU fuji -> annex okinawa -> vassalize tachi in that order. Doable by ~1420. The main advantage of uniting japan is losing the horrible -50% naval forcelimit modifier.
The biggest disadvantage, when you unite Japan is ihmo, that you can't dumb all your Infamy to Japan (=Emperor). On the other hand, this feels like an exploit to me.