Alex, there is a negative correlation between risk/reward and probability of success - https://steadyoptions.com/articles/post/general/risk-reward-or-probability-of-success-r91. Fly is basically a narrow IC. IC can also have different risk/rewards, depending where you place the strikes. The wider the strikes - the higher probability of success, and worse risk/reward. IC has good risk/reward, but to book those fantasy land returns that you see on the P/L chart, the stocks needs to stay exactly at the middle strike, while IC can have much wider profit zone. As you mentioned, it ultimately comes to risk management.
You should never look at those trades as income generators. Bank interest is income generator. Dividend is income generator. Those trades are NOT. The fact that you get a credit, is irrelevant because you have no guarantee that this credit will stay in your account.
When you say flies - could you please clarify which flies do you mean? On indexes? Or stocks? In context of earnings?