Their production sounds very very slick and glossy. It's like the music equivalent of eating pure sugar and it was a barrier to enjoying the song.

However eventually their songwriting won out.
I can't go on about the wonders of flowing melodic writing and then not love Dancing Queen which is a perfect example of a continuous melody line from verse to chorus. A pop masterpiece

(by the way,I think Dancing Queen is significantly better song than Knowing Me, Knowing You at least from a melodic point of view. Te verse seems underwritten and the song only catch fire during the chorus, although there is melodic flow, there's more repetition compared to Dancing Queen and it doesn't have the sense of build up to the soaring chorus).

Or love the smooth multipart pop song songwriting of The Beatles and then not love Mamma Mia which has 5 distinct parts that is join together without sounding like a medley (which is something even The Beatles failed to do a lot of time).

Or love the dense and atmospheric arrangement disco/synth pop of No. 1 In Heaven and without appreciating the arrangement of Voulez Vous album.

In fact ABBA transition to disco seems like they are the closest analogue to Sparks in the 70's.

I give them a 2 star rating though, I think what prevents me from elevating higher is that I do feel there is reasonable gap between the hit singles and the album track. While there are deep cuts (I am a Marionette is a masterpiece and a few deep cuts from Visitors) I often find myself listening to individual tracks over entire album.