The Allflame Ember does not replace the modifiers and it only replaces the monster type. The buff from the Allflame Ember goes hand in Path of exile currency hand with the modifier, giving you a chance to improve the loot of certain mobs.

Here are some of the best Allflame Ember combinations you need to look out for:

Yellow modifiers with increases to quantity or rarity of items found and an Allflame with buffs to currency items, Karui Tattoos, and Scarabs. Allflame Ember of Rats or Harmless Frogs and any yellow modifiers that want to increase pack size. The strongest monster in packs gets drops converted into "X" and Allflame Ember of Anarchy
While these specific Allflame Embers are quite hard to find, you can either buff your atlas passives to get more Allflame Embers in maps or buy them in the market in bulk.

Necropolis Grave Crafting Guide
Here's a simple rule you need to know when crafting gear – a gear can only have three prefixes and three suffix mods (uncorrupted). This is the baseline for all gear crafting and Necropolis aims to manipulate the "randomness" of POE buy currency rolling stats into a gear.