From what I'm seeing:
BL: has the easiest early game roster, but has a hard time with Hunting by Daybreak and has easily the worst endgame chapters. Have fun with all the ballistas and ambush spawns/siege units. You do benefit from relics more on this route, however, considering you can potentially get the most on this route (including Failnaught).
BE-E: has the hardest early game roster, but lacks the infamous post-timeskip chapter that everyone else gets. Endgame can be fairly difficult, but you do get Raging Storm abuse and a unit with both Mastermind and Counterattack which can make a good difference. You also lose access to quite a few paralogues here.
GD: is sort of inbetween both BL and BE in terms of early game, has nice rallies available early on. Easiest access to Thyrsus, and Reunion at Dawn is the easiest of the three since you get a free flying unit with Ashes and Dust gambit. Endgame is definitely the easiest of the four, the units don't move unless you attack them or get into their range, and there are not many ambush spawns.
BE-C: has the same early game roster so it's just as hard, but on top of this, you lose your lord and another powerful unit, and has by far the hardest time with the first timeskip chapter. That being said the lategame is similar to GD and the final map is moreso annoying than difficult I'd say. Miracle still sucks and can force divine pulses if unlucky.
Difficulty-wise I would say GD > BE-E > BL > BE-C. Probably not by a whole lot tbh, especially if you're using DLC to grind, in which you can build up your units pretty quickly patching up the underleveled state of your characters.