おまけに

お

ま

け

に

Meanings

usually written using kana alone

Adds an additional negative or undesirable point to a situation. Often used when listing complaints or unfortunate circumstances.

その上うえN3
その上 is neutral and can add either positive or negative information, whereas おまけに almost always adds a further negative or unwelcome fact.
しかもN3
しかも is a slightly more formal connector that can introduce positive or negative additions, while おまけに leans toward complaining about extra bad news.
それにN4
それに is a casual, neutral way to add another point, whereas おまけに specifically emphasizes that the added point makes things worse.
それどころか
おまけに simply adds an extra fact on top of what was said, whereas それどころか implies the added fact contradicts or far exceeds the first statement.
そればかりかN2
おまけに is a casual, conversational way to add an extra (often unwelcome) point, while そればかりか is a bit more formal-sounding and can be used in writing as well.

Applies to each form listed below

rarely-used kanji form

Derived from おまけ (bonus, extra) + に (particle), literally 'as a bonus'. The negative nuance developed from the idea of an unwanted extra.