Question: The Curse:

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> **Clockwork Curse**  
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>  All Attack Rolls the cursed target makes replace their d20 roll with a 10 instead of rolling, as per the effect of Clockwork Amulet.
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Extra Details:

* This curse is expected to apply for at least a full in-game day.
* Encounters for the day will come from the Forest or Coastal Encounters (5-10) tables in Xanathar's Guide to Everything.
* The character will be part of a 5-player group.
* The character's attacks are their ideal method of dealing damage.

The Goal:

* This curse is meant to be a setback or gameplay-shift to the player, not a crippling of the player.
	+ I want the player to spend time affected by the curse to work with it or work around it.
* Since the players can decide not to remove it, the beneficial aspect of it should not be too strong.

The Question:

Is this curse too detrimental that it imposes more of a penalty than I expect? Is this curse too beneficial in the long-run? Or is it reasonably fair?

Answer: 
Against an average enemy this would be a buff not a curse
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Rolls are generally balanced around requiring an 8 to succeed in something you are good at, so always rolling a 10 is a buff (on average, assuming the player invested in the combat stat).

This does mean that against certain high AC enemies this player will never hit.

So how this will effect your game depends firstly on how many high AC enemies you chuck at the party / player, and how they will react to not being able to hit things. Some players will welcome the challenge and have to think of new tactics, some players just want to cause damage and put very little thought into it.

You know your players (hopefully) better than we do, so you would have to judge this for yourself.
Situational. 
Sounds like it would make hitting most low to medium enemies easy, but make it impossible to hit a boss or demi-boss. 

So useful for a front liner who will be wading through canon fodder mobs. 
Not directly useful against a armored one or boss, but silver lining. Against those, it would encourage the player to use the "help" action to help their team hit the boss.