I remember stumbling onto this one a couple of years ago and basically going "What the heck are the Groovie Ghoolies and why does this exist?". I eventually saw this one 2 years ago (my review here:
https://letterboxd.com/mjkallamthanam/film/daffy-duck-and-porky-pig-meet-the-groovie-goolies/)
and....yeah, it was as bad as people have kept saying it was. The print I was watching was a reconstruction and the print quality significantly dropped in the second which only really added to it's "what the-"ness.
There are actually a couple of good bits here and there (my favorite being the "arrows hitting the typewriter" bit) and some actual good poses here and there and the live-action sequence is actually pretty good (but it might have not been made for the special so it probably doesn't count) but...that's really it. And for every single positive I can come up for this special, there's about 1000 more issues I can namedrop (As Wiley207 stated, one is that Mel Blanc's voice sounds terrible (his basterdization here being only matched by his role in those early 40s Columbia toons)). Though I will say that this is really just on par for 70s Filmation, it's just the presence of the WB characters that makes it worse (if they didn't appear in this at all, this just would have been seen as another crappy 70s cartoon and that would have been that but no...).
I feel bad for a lot of the former WB cartoons mainstays that worked on this mess; Len Janson (who wrote this), Dale Hale (doing the storyboard), Don Christensen (doing the art direction), Tom O'Loughlin and Boris Gorelick (doing the backgrounds), Ed Friedman, Larverne Harding, Virgil Ross and Hank Smith (doing the animation). I bet they felt like a rat working on trash like this. (Incidentally, Lou Scheimer actually worked on a WB Golden Age project as well...the Gateways to the Mind special in the late 50s!)
Weirdly, Chuck Jones made a similar special to this in 1978 with A Connecticut Rabbit in King Arthur's Court. It also has Daffy Duck in the King Arthur role and the pacing is also awful (though the animation at least is a lot more competent and well done than this). It's roughly on par with this special in terms of entertainment value.
For the record, according to Lou Scheimer this special came out due to their deal with WB (likely this one mentioned here:
https://www.worldradiohistory.com/h...BC-IDX/71-OCR/1971-02-01-BC-OCR-Page-0051.pdf).
And here's also a 1994 Animato article talking about this one:
Animato! Issue 29 (Summer 1994) : Free Download, Borrow, and Streaming : Internet Archive. Incidentally, a Newspaper from St. Joseph, Missouri from December 16th, 1972 calls this a "delightful animated feature"!
(By the way, if you want to see what a Filmation WB short probably would have looked like, watch Quacker Tracker from 1967. It contains similar Daffy Duck designs, that "feels like it's missing shots that never existed" syndrome where characters randomly teleport for no reason other than to save money, 2 animators that worked on this special (Ed Friedman and Virgil Ross) and is cowritten by a guy who did art direction on this special (Don Christensen). Though the backgrounds are much better and the gags are slightly funnier (though only slightly, it's still a 2/10))