Building flat AP instead of MP or Scaling AP

I'm a support main, but I play extremely aggressively, even to the point where I sometimes carry earlygame and push back champions that are usually thought to counter Soraka, therefore getting my ADC much more farm than the enemy ADC. Usually when I build, I go for flat AP everything, and completely ignore MP and Scaling AP. My thought is that the enemies are likely going to think that my ADC is the main threat, and so will ignore me. It also seems like I'm very powerful earlygame, but then sort of drop off mid to lategame in terms of damage (at which point I can focus on healing, which gets very powerful lategame with my build).

Is this a good way to build Soraka, or am I missing something that could be a total catastrophe for me? What would be the most effective way to counter my build, and how would I overcome said counter if my ADC doesn't destroy the champ that decided to go full MR in my lane?