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: 
This punishes player fun more than character mechanics
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Dependant on the AC of your enemies this curse is either a massive buff due to never missing, or a massive penalty due to never hitting. Either way it takes away the uncertainty and suspense of combat by making the result a formality.

In my experience a lot of the fun of combat is encapsulated in the thrill of the unknown. This curse takes away that unknown by removing the agency from a cursed characters turn. The curse has two states:

1. 10 + Attack modifier >= enemy AC. The player knows they are going to hit so every turn they can skip straight to rolling damage. No need for clever tactics or positioning to gain advantage or cancel a disadvantage. The character is effective but boring to play.
2. 10 + Attack modifier < enemy AC. The player knows they can't hit. There is no point trying so they are instead forced to play a support role, depending on the type of character they may quickly run out of things they can do in this role. No interesting options and a lack of effectiveness makes this character frustrating to play.

In both scenarios **the mechanical penalty to the character is not as significant as the penalty to player enjoyment**. In my opinion that makes this a bad curse.

A good curse challenges both the player and character to find a way to overcome, cure or circumvent the limitations imposed by the curse. Something that forces them to change their typical combat style or think outside the box. This curse doesn't do that. It is either good or it is bad, and there is (almost) nothing the player can do about it.
Seems like a serious debuff
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In terms of what the character can actually do -- you roll a 15 or over like 25% of the time. That makes for a hard fight, but you can burn resources and win it. This curse makes easy fights trivial, and hard fights impossible. You might be able to, as the DM, design around this -- make things harder by giving more low-level enemies, and totally avoiding high level enemies. But that sounds really tedious. 

Also, how will you deal with advantage? I suspect the expected value of a roll with advantage is about 15. If you give them a 15 on an advantaged attack, it might be more manageable (also more fun).