It is far from strong, and feels even weaker

It is great when it works, but cannot always be used, does not do anything half the time, and costs your reaction even when it did nothing.

Limitations

• Positioning: Protection can only work if you have an adjacent ally, while Dueling or Defense does not need cooperation from your allies
• Cost: You have to spend your reaction to use it
• Rare success: More than half the time you won't see any effect (see below)

Disappointment

Why would you feel disappointed? Let's go through all the possible rolls (assume the DM usually uses the red d20, the green only comes out for Advantage / Disadvantage)

• Both rolls high; the target is still hit, disappointing
• Both rolls low; it would not have hit anyway, disappointing
• Red low, green high; the disadvantage did not matter, the usual die would not have hit anyway, disappointing
• Red high, green low; you actually helped the ally! But look, the next attack you could have prevented is a natural 20, you should have used it on that one, disappointing

Obviously the last one does not happen this way every time. Still, even if you succeed, each time an enemy rolls higher later in the round, you will feel regret not preventing that one instead.

Calculation

I will assume the enemy has 55% hit chance against the protected ally, 45% against you (you must have a shield for Protection), and compare using Protection on your ally, or Defense on yourself.  

Protection

If the enemy has 55% hit chance before disadvantage, it goes down to 30.25% (0.55 * 0.55), 24.75% gain.

Defense

This decreases the enemy's hit chance from 45% to 40%, a 5% gain.

Comparison

Protection can only be used once per round, so if you are attacked 5 times in a round, Defense is better. If you consider that Protection needs careful positioning and will not be usable every round, they prevent about equal amount of damage to the party per encounter¹.    

The benefits are equal, but not the costs. Defense is always on, but Protection consumes your reaction, preventing better uses of it².

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1. Preventing 10 damage to the Wizard is more useful than to the Fighter, compared to their max HP. However, at least in my experience, the front liners run out of Hit Dice well before the back row casters, meaning it is actually better to spread out the damage.    
2. Opportunity attacks, Riposte (BM maneuver), Sentinel (feat), Counterspell (spell) etc.