Scion isn’t a full caster. So you’ll miss out on the level 7-9 spells. Also there’s merging spellbooks vs taking them individually to consider. and, the fact that demons have decent spell resistance. So, going full caster with the mythic abilities/feats is certainly helpful in ensuring that those spells actually land. That’s also why I lean towards merging spellbooks with any full caster. As, it boosts your caster level up to a potential 30 at character level 20/mythic 10.
There’s also the enduring spells situation at caster level 25 that has been brought up a few times in the alpha forums. If you have both of the enduring spells feats and a caster level of 25, your cast extended per round spells(like displacement) will last 24 hours instead. So, that’s something to consider when multiclassing. The only real exception there is the Hellknight Signifier prestige class, as it merges spellbooks at level one(bugged in alpha). In most other cases, you lost a caster level and the merge happens on level 2.
If you take a look at my posted Lich build, I lost 3 caster levels by multiclassing. Which means, I wont get the per round buffs extended until caster level 28. Now, this was all based on the alpha, and they have talked about changing how mythic spellbooks are combined with your existing spellbook, and how the casting stat is affected. I’m not sure if that will affect the caster levels though.
In alpha, at least. I found that the builds that you could use in kingmaker worked very well in WotR. The extra classes add a lot more customization to your builds. However, with the exception of mounted combat; the new classes play very similarly to a multiclassed build as it is. Typically offering a mix of two other classes abilities without the drawbacks to multiclassing. For instance, with Magus, you could now take the Armored Battlemage sub-class. And ignore the scaled fist level. Then you’d still get your capstone ability. It trades the weapon enchantments and puts them towards armor and protection instead. Plus, there’s plenty of ways to tweak your weapons with spells in WotR anyways.
Anyways, there’s lots of things to consider.