Guides don't always work for everyone because they might not want to use them or don't/can't be fucked to bother with units that just don't stack up to the newer ones.
Not everyone is a star at the maps, and they're not worth it imo, but okay, tell people to carry crappy units who are only good if you throw 20-22k feathers at them to get their refines and are otherwise outclassed by the four or five stars new players need room for in the barracks, also LhBs don't give enough rewards to the effort it takes to clear them only 40 codes is a damn joke for two maps when you can literally sit at Tier 18 with the auto dispatch feature in Aether Raids and get more codes there than you'd ever get from wasting time doing the LhBs, especially taking into account other sources that you can also get them, if only trait fruits and orbs were that souped up in distribution, then people might get somewhere as low end investors.
Most new players probably don't whale on heroes like you clearly do because Grails are nowhere near as bad as orbs/IVcardos to obtain, which I as a semi FtP know far, far too well my orbs never last enough for units I spend for to get needed units to even progress in Heroes or SI when I have a copy I could easily give up but can't due to "Oh I might need that for those stupid LhBs or arena/AR bonus."
That said I don't waste my grails on old useless units anyway, and if you're really hamstrung for resources you have to make the call whether or not it's actually worth the three or four orbs per map in some cases (spoiler alert it isn't), or you just don't waste your time with it. That is where the FtP guide is important. They know what it means to be truly hamstrung with a shoe string heroes budget, you obviously don't. Especially as a long time player, because let's be real, most long-term players forget how terrible some of the GhB units really are, and they aren't even used in guides anyway. Even in the LhBs I'd wager, especially as 3 star units, so turning them into manuals is practical because you can grind SP as a four or five star far more efficiently than a three star, liable to wif at the tiniest of hits. I will freely admit that yes, I didn't train and raise my 3 star Reflet and went for the four star. It wasn't worth the 2k feathers at that time because I was a new player without half of the decent units they get now.
Also fyi none of the old grail units have been used in any guides I've seen, so make of that what you will.