Riftmaker is usually used in long combat and it is only compatible with some particular Champions, for example: Akali. With a nerf in resistance, Akali need Fleet Footwork instead, and Riftmaker provides her continually increasing damage in every clash (up to 15%) and convert into true damage.
In another word, there are many Champions seem to be suitable with this item like Swain, Mordekaiser, Sylas, Rumble,… However, in order to reach to full potential of this item, you should build it secondly or thirdly.
About Everfrost, players do not favor this item much, inspite of its quick-cooldown slow ability. But, it can still be useful with TF, Ahri, LeBlanc or Morgana.
Everfrost is a late game item, so don’t worry if you have a quite hard early. Especially, if you have five Legendary Items, you will gain 155 AP as bonus.
Although they are both strong items in late game, but clearly, with AP builds, Everfrost and Riftmaker are not essential. They need some modifications in order to reach their full power, for example, make it faster for Everfrost to reach 155 AP bonus.