Strange, I never thought that Ftr19/Wiz1 was severely sub-optimal. One level of spellcaster added to a near-full martial character generally worked well in 3e: you sacrificed 1 point of BAB and a few HP for a bunch of spells that could be really useful. Obviously, you needed to pick good spells... damaging spells were bad choices because by that time a 1st level energy spell may be to weak, but control spells or utility spells could make a very useful addition.
Also Wiz19/Ftr1 (or another martial class) could work decently.
What usually didn't work well at all in 3e were multiclass PCs with about half levels in one spellcasting class, and the other half in other class(es). Actually also 1/3 levels, 2/3 levels etc... basically spellcaster levels worked in multiclassing only when you took either very few levels (1 or 2) of a spellcasting class or almost all your levels were from the same spellcaster class.