Yeah with the cube letting you get RoRG if your build needs it or unity/CoE/zodiac, F+R is basically a must in every build since it gives so much damage. I wonder if they have plans to fix this. I mean it's not a huge issue but it's pretty dumb design. I guess if they made a single ring that somehow conflicted with F+R but also provided a large amount of damage, it could be remidied by having the new ring+CoE with a Unity/Zodiac/RoRG cube or F+R depending. I guess a way would be to give you a stupid damage buff for as long as you haven't used a builder in the past 5secs(or 10seconds even), which feels artificial as shit but would conflict with F+R. This way, if your build needs a primary to generate ressources, you use F+R, if you don't, you use that new ring+CoE. For it to be even though the ring would need to be like, +75%damage, multiplicative? And it'd still be less damage than F+R with CoE cubed if you can do that(WDs do at least until 65ish, might need unity after though), but it'd help builds that can make use of other rings and need unity to progress.
Not sure why you can't roll 2piece set items with the cube though. I mean I get it if they could roll ancients, as it'd let you to reroll for ancient weapons from the hallowed set or other class specific sets like majuma, danetta, bulkathos, shenlong and so on, but since they nerfed the set reroll to not allow ancients, I don't see why they don't let you use it with 2pc sets. I guess they don't want you to complete a set in a single reroll or something, but it's pretty annoying when you're trying to get some of the sets, and especially F+R since there's like 20legendary rings since you can't even use the upgrade rare recipe that efficiently. Maybe they should have another cube formula with double or triple the mats but only for 2pc sets.