Pact of the Chain is well balanced without the variants.
They get a good invocation in Tasha's to further buff the familiar.
The variants are overboard. Magic resistance alone is very good.
Imagine my surprise when I saw this 3 year old video in my recommends tonight that argues that the imp's magic resistance doesn't transfer over. According to the YouTuber, the magic resistance only applies as part of an infernal contract with the imp. If you gain the imp familiar from the find familiar spell or warlock invocation it is only a spirit that takes the form of an imp. Most relevant part starts at about 7:42 into the video.