I have been mixing ammo for well over 30+ years with no problems at all. My main reason for doing it is to get rid of old ammo in my stash to replenish with new ammo.
For instance, I have some 2.5 year old 9mm OATH ammo and TULA ammo I have had in my ammo drawer that I will be shooting this weekend. I will be loading magazines with it, mixed up. I will be replacing it with Hornady Critical Duty and WWB.