It appears to be common knowledge that split-stat builds for fighter/casters and rogue/casters just aren't effective.
Fighter/casters have to use staves for decent magic damage and Warfare skill usage, but those Warfare skills are still resisted by physical armor, so your spell damage doesn't contribute towards those skill's resistable effects. Rogue/casters can use Staves as well for damage, but have no access to all the Scoundrel abilities that require daggers to activate.
The archetype that players will want to use here is very clear, but the game is asking you to optimize away from those classic archetypes to be effective - no sword-wielding or backstabbing caster for you, unless you wish to lag behind your companions.
To solve this problem, we need to address three core problems:
1) Splitting STR/INT and FIN/INT means your spells and your melee attacks will scale far behind other specialist builds.
Any build that is a "split class" shouldn't be as good as either dedicated class - it trades outright power for versatility and synergy. But rather than have a character's skills scale to 50% effectiveness of a dedicated class, we should shoot for something more workable - for example, 75% (exact number is where more detailed balance testing comes in).
We can accomplish this by adding two new Talents:
Battlemage: STR contributes to spells that scale with INT at a rate of 50% per point. INT contributes to weapons that scale with STR at a 50% rate.
Shadowblade: FIN contributes to spells that scale with INT at a rate of 50% per point. INT contributes to weapons that scale with FIN at a 50% rate.
This requires that the player use one of their talents to cement their hybrid status and moves both spellcasting and weapon damage closer to usable levels. In theory, this would make your scaled values 75% of a dedicated build, assuming that's a good target percentage.
2. Non-staff weapons do not do magical damage, forcing hybrid builds to split damage types.
This is obviously not effective. We need some way to allow for magical weapon damage. Mods exist for elemental versions of weapons, but you could argue that they break the flavor of the game's weapon design. What does it mean, after all, to have a sword that does only fire damage? Does that sword not have any steel in it?
One approach is to include skills that allow you to "imbue" your weapon with elemental damage. For instance, a Warfare 1 / Pyrokinetic 1 skill (1AP) - once invoked, it transforms your weapon into a flaming variant for X turns, effectively swapping its physical damage for magical damage. To make this work best for true split-class players and not just vanilla fighters/rogues, the duration of the transformation could scale based on the elemental school level.
3. Warfare and Scoundrel abilities resist based on Physical Armor.
The solution for this is to re-work these abilities such that they resist physical if you are using a physical-damage weapon, and resist magical if you are using a magical-damage weapon. If that is too broad, then make them switch damage resistance only when the player activates the Imbue ability listed above in #2.
The game currently provides decent options for players that are primarily fighters or rogues that dip in caster skills for non-scaling spells, or casters that dip in Warfare or Scoundrel for skills that don't depend on physical damage resistance. If we can get solutions to the three problems listed above, we can finally have what most players would like to play: a true 50/50 hybrid that doesn't shame their party.