"Be moderate in order to taste the joys of life in abundance."
~ Epicurus ~
I have observed two possible responses to telling someone they are amazing and doing a great job. The first is that they heighten their integrity an additional notch, walk a little taller, and become scrupulous about continuing to earn the acknowledgement.
And the other, sadly, is to slip a little and become less scrupulous figuring now they have won us over that they can do no wrong.
I avoid enthusiasm backlash by keeping my compliments right-sized.