An Iron Condor is usually constructed with a short OTM call, OTM put and further OTM long calls and puts (for hedging). What does that tell you? The risk of the shorts getting ITM is lowered by the OTM amount. So lower risk, lower reward.
The iron fly is short ATM calls and puts. So just comparing with the iron condor, higher risk, higher reward. That's all.
Also, the iron fly is just no different from a normal all-calls fly or an all-puts fly. Dont be fooled by the credit. The difference in strike price less the credit is what the all-call or all-put fly will cost you. Synthetically, they are the same.
What is your market view? If you believe the underlying will just sit ATM, then the Iron fly or regular fly is a good strategy with limited risk (due to lower cost) and good reward. Flies are low probability high reward trades. So dont expect many of these to go in your favour.