I think it is even the opposite, .....
... One of the first things you read is, that Shiva grants Boons without judgment. The worst of demons get the best of Boons if they pray hard enough. God shows us by example "do not judge" which is taught also in the Bible, but ,,,
This is not the complete story. Although Shiva is "BholenAth" and grants boons impartially to demons, VishNu never does that.
In fact, VishNu takes avatAr to destroy the demons that received boons from BramhA and Shiva owing to the austerity they performed, and became a menace to the Universe.
That is VishNu's promise in the Bhagvad Geeta, chapter 4 :
Whenever dharma (righteousness) declines, and adharma (unrighteousness) increases, I appear to destroy the wicked[ness] and protect the innocents and Saints.
and there is a 3rd reason which He did not mention in Chapter 4 -- He appears in the hearts of devotees to simply give them His association and uplift them -- this He can do anytime, even today.
As an avatAr, He appears on earth - that is when He destroys the demons who have appeared on earth. This is what He did as KRshNa , RAma , ParashurAma , NRsiMha ... avatArs.
Demons do not get away with boons for long.