logic would dictate that the normal drafter would draft his first flex player in the 3rd round because it is impossible to pick him before that. the first 2 picks would fill a required starting spot. so bush should be considered a 3rd round quantity. this logic already assumes a disadvantage
if that is the case, then you are forced to play your 1st rounder as a third rounder, with no deviation.
assuming same drafting pattern of RB/RB/RB/WR/WR/WR/FLEX
in that case your lineup would be...........compared to opponents
.............RB 2nd.........................................RB 1st
.............RB 3rd..........................................RB 2nd
.............WR 4th.........................................WR 4th
.............WR 5th.........................................WR 5th
.............WR 6th.........................................WR 6th
.............RB 1st.........................................WR 7th/RB 3rd
fine. they are the same, but bye week consideration. and considering you may get opponents with full compliments of teams due to dissimilar bye weeks equates to this (assuming you picked the first 7 picks with different bye weeks)
RB2nd bye.........RB3 bye.......WR4 bye....WR5 bye....WR6 bye....bush bye (RB1)
RB3rd..............RB2nd..........RB2nd.......RB2nd.......RB2nd......RB2nd
RB7th...............RB7th...........RB3rd........RB3rd........RB3rd.......RB3rd
WR4.................WR4.............WR5.......... WR4..........WR4.........WR4
WR5.................WR5.............WR6...........WR6..........WR5.........WR5
WR6.................WR6.............WR7...........WR7..........WR7.........WR6
RB1..................RB1..............RB1............RB1..........RB1..........R
7/WR7
these examples show that taking your flex in the 1st forces you to use your 4th-7th round RB 2-3 times a year with no opportunity to play lesser talent at better mathups (what if your RB3rd (rbrown/edge) plays the #1 run D and your WR8th (plax/ward/coles) plays the #32 passD, tuff luck) ***using FBG ADPs***
while your opponent would never have to play any RB beyond the 3rd round (you are forced 2-3 times at 4th-7th) and forced to play your WR7th 3-4 times while your opponent would only need to during 6 byes.
so in conclusion,
there is a definite disadvantage to taking bush in the 1st, you will always be at least 1 round in talent (maybe more) during bye weeks and the inability for flexibility during the course of the rest of the season will hamper matchup play possibilities.
thats worth about 6 cents