IMO, they are funny mainly when they are playing "straight".
Carrey in "The Duck Factory", "Once Bitten", and his straight bits in "In Living Color" were really good, because in those, he was the Straight Man!
I even liked him in the beginning of "Batman Forever", before his transformation into the Riddler.
In "The Truman Show" he was great, even.
The only roles where "Carry being Crazy" worked well was "The Mask" and "The Man On The Moon".
Like many authors who need a strong editor, Carrey needs a strong director, someone who has the strength, the vision, and especially the CLOUT to tell him that he's better when he's toned down.
Similarly, I've never really enjoyed the "Robin Williams on Cocaine".
Everything that "Mork and Mindy" did, both "My Favorite Martian" and "Third Rock" did *better*, mostly for being toned down enough that the insanity had time to be appreciated. (I didnt help that no TV show in the 70s had even heard of taste, let alone exhibited any.)
The only "crazy Robin Williams" roles I've really liked were his Genie in "Aladdin" and his (uncredited) Ray DiTutto in "The Adventures of Baron Munchausen", and also maybe "Good Morning, Vietnam" (and in that one, mainly because the insane high energy voice was obviously a role that the character would adopt for his "on-air persona", rather a role that Robin Williams adopted for the character).
Well, and his physical comedy in "Popeye" was brillant, and so perfect that now whenever I see the old cartoons, I mentally subtitute Williams version of the character in.
OTOH, his roles in "Toys", "The Birdcage", "What Dreams May Come", "Insomnia", and "The Final Cut" I enjoyed, because, again, while they had an edge of insanity, they were Toned Down, so we got to enjoy the edge, rather than being assaulted by the noise.