5.0 with a 4 cyl. t5 question?

I have a 89 mjustang , 5.0 with a 4 cyl. t5 temporaraly instead of a v8 t5. I have read up that its ok to use an in fact some people have gone in the 12s with this set up.I know the input shaft is smaller and the synchrose are diferent, why am i told that i might hate this set up? can i get some input?