This is just not true.
Assassins generally rely on flat AD and lethality/penetration.
Marksmen go speed/crit and flat AD.
Juggernauts typically flat AD or on-hit.
Yes multiplicative scaling, for marksmen in particular, tends to come from secondary stats, but many of the benefits of the unique passives are worth far more than the loss of value from the cost of the secondary stats. With most items providing other forms of damage that they still benefit from. The only exception to this that I can think of are the
items which ONLY provide secondary stats. And a full attack speed/crit marksmen is useless, you still need to build flat AD on them.
But let's lay it out this way, when
or
are strong, all of these classes can justify picking them up. When one of those are weak however, one of the classes feels a substantial hit to their ability to effectively fill their role, while the other classes still have their tailored items to fall back on, even if they're not able to use one of the powerful items designed for a different class.
Then you have other items which are just made for all of them and none of them,
provides the flat AD for everyone. CDR for the juggernauts and assassins. Lifesteal & spell vamp for the marksmen and juggernauts. Burst dampening for the assassins and marksmen. That's not to say that DD is OP, only to use it to demonstrate that in fact ALL of these classes absolutely can use the same itemization. As I said in my OP, they have no business to be doing so when they're also intended to act as counters to each other.