Both have different opportunities
I am tempted to answer twice, as Uke and Nage have interesting aspects to be learned.
When I am Nage, I must be carefull not to use muscle / strength opposed to technique which multiplies strength to a level that can injure my training partner if I don't take time to feel the amount of movement needed to create a safe training environment.
On the other hand, as Uke, I must follow the flow of energy without makeing it too easy, or get too far ahead of the technique, or too far behind which will cause a greater chance of injury to me. Uke must create enough offensive resistence to make training effective while not roaming into the area of strength beyond your partners capability, while being prepared to ride the wave of the technique in a safe manner that allows for learning protection while being manipulated, or thrown.
Dillemma .... Pair of Ducks ... unlike a Paradox ... or a pair of docks.
How about another answer.
The hardest thing about being Uke or Nage is sitting on the sidelines either waiting for your turn, or being caught up ... Like in one of my dizzy spells where the floor and walls are moving like a ship in a hurricane.
Yeah, I think sitting on the sidelines in harder.