For missiles launched from Russia that would pass over Europe on their way to the US, or pass over AEGIS class ships in the Arctic Ocean the acceleration rate and predictability of a missile launched from Russia is a very big deal... and as mentioned no minor correction of trajectory as the missile starts to climb and accelerate would be difficult for most interception systems.
Serious course changes however would require serious structural strength that most long range ballistic missiles lack but also it needs to know where the threat is coming from which requires sensors to detect incoming active radar homing threats... which adds weight too.
Mid course interception is made much more difficult with a high acceleration missile that actively manouvers to be a difficult target.
The cost would be extra structural weight and extra components to give the missile an awareness of potential air defence systems trying to intercept it, but I would suggest well worth it.
The situation for China is even worse with interceptors likely based in South Korea and Japan as well as the Pacific ocean.
Obviously something Bulava has been adapted for, and Poseidon and Thunderbird bypass completely.