Multi-classed also lost hit points (you round down).
I agree the multi-classed characters were reasonably comparable to single-classes though I think they operated at closer to 90% in each class than 70% -- for the levels where they were able to advance, anyway.
I never found school specialisation worth it when I played a Magic-User; you surrendered too much so that's probably a play style thing. Armour restrictions worsened you compared to a Fighter, but not compared to the Magic-User. Weapon Specialisation loss can be a big deal if the optional rule allowing it is in use in the first place. Typically, it is used.
Compared to a straight Fighter, a F/MU
- needs an Int score
- Loses hit points
- Gains improved saving throws vs. Rods and Spells
- Loses weapon specialisation (if the optional rule is in use)
- Gains extra non-weapon proficiencies (if the optional rules are in use)
- Uses the to-hit chart one level lower (typically -- it varies of the course of the career)
- is very restricted in armour (elven chain or magical defences only e.g. bracers, ring, and cloak)
- can cast spells as if he were a M-U one level lower than the straight Fighter (typically -- it varies of the course of the career)
- Can use items restricted to M-U
- Has a cap on level advancement
Compared to a straight Wizard, a F/MU
- needs a Str score and can take advantage of a high Con
- Gains hit points (especially if Con is 17+)
- Gains improved saving throws in most categories
- Gains more weapon proficiencies
- Much better at attacking (6th level Wizard is the equivalent of 2nd level Fighter, 9th is 3rd)
- Can wear some armour (elven chain)
- Can use items restricted to Fighters
- Must be a Mage rather than a specialist
- Casts spells as if he were a level lower than a straight M-U (typically -- it varies of the course of the career)
- Has a cap on level advancement
