I think the biggest "issue" with additive is that it can be complex to program at its most low-level implementation. I'm thinking of stuff like the K5000 where you could not only set the initial volume of a partial, but every partial had an envelope to control volume. Extremely powerful, but on more complex patches that's a lot of envelopes to think about.
On the other hand, "simple" additive with relatively few partials is actually pretty easy too. (The Hammond B3 and other tonewheel organs are essentially additive synthesizers.) A lot of software (your Alchemys and Razors) is built on a simplified version of additive too, or uses additive techniques to perform "resynthesis", or has other tools that "simplify" some of the additive editing.
On the other hand, "simple" additive with relatively few partials is actually pretty easy too. (The Hammond B3 and other tonewheel organs are essentially additive synthesizers.) A lot of software (your Alchemys and Razors) is built on a simplified version of additive too, or uses additive techniques to perform "resynthesis", or has other tools that "simplify" some of the additive editing.