In conversation, I just said:
Tu es sans pitié, dis-moi... En même temps, elle n’aurait pas autant envie de nous rattraper si on n’était que douceur avec elle.
Essentially, I wanted to express the idea of:
{literally}: You give her no quarter. Then again, if we were nothing but sweetness/gentleness towards her, (she would just content herself with the current 'her' without looking to improve herself and) she wouldn't be so keen on catching up with us.
{more naturally}: You give her no quarter. Then again, if we were treating her with kid gloves all the time, she wouldn't be so keen on catching up with us.
The context is: "In training the residents at our hospital, we can't afford to be constantly indulgent and lenient towards them, as if we don't have an ounce of strictness in us. Little pep talks here and there wouldn't go amiss."
I'm concerned that the phrasing "n’être que douceur" might not get across the intended meaning but rather be interpreted as "être (tout sucre), tout miel" with the meaning of "se donner une apparence de douceur" – which is effectively the equivalent of "be all sweetness and light" with the meaning of "behaving in an overly pleasant and friendly manner that doesn't seem genuine".