Raid eidolons are some of the worst, don't use them unless you have no other option. Story exchange eidolons are just as bad but they can at least provide you with battle buffs.
It'll take less effort to max out a R eidolon that'll have decently good stats for beginners to use than it would take to level a raid eidolon to be worth using and only be slightly stronger than that R despite being 30 levels higher.
For example:
Werewolf, a dark R eidolon, easily farmable from certain story missions, 900 ATK at Lv60 (max) and gives 30% Dark ATK boost when maxed if using as main.
Dark Disaster, the dark SR raid boss eidolon (or any disaster works, they all have the same stats), 1000 ATK at Lv90, that's 30 levels worth of enhancing just for 100 more ATK and even at max break, it's only 20% ATK and HP. And this is assuming you can even manage to fully break limit it before you get something else worth using.