Subspace Emissary had dreadful, slow-paced sluggish platforming, idiotically designed bland forgettable original bad guys, lazy "puzzles", spending your time mindlessly beating up a gauntlet of enemies for quite some time, limiting characters to play as, extremely generic stages (oh hurrah, let's run through iconic Nintendo location "The Caves"), littered with generic, original enemies that no one gives a crap about (do any of you remember what a "Cymul" is? Or a "Gamyga"?) in game that has crossover franchises rich with recognizable enemies (Smash Run is how you do it properly), a story that made absolutely no sense and wants you to take it seriously for hell's sake, bland, silent protagonists, the length of the mode that dragged on far too long, and the worst level of the game, The Great Maze, which serves only as a gigantic way to pad out the game even more with locations you've been through and refighting bosses again.
World of Light is a slow-paced, sluggish mode that dragged on for more longer than it was necessary, was extremely repetitive in how you fight spirits over and over again, had idiotically-designed, bland forgettable original bad guys, the cutscenes had mediocre delivery and unnecessary voice acting which wasn't taken advantage of, also has a ton of difficulty spikes littered all over the maps especially with many of the Legend spirits, some characters were fucked because they were unlocked far later in World of Light, and a padded out repeat boss gauntlet fight near the end of the game.
Honestly, I liked neither mode all that much, wanted them to end quickly, etc. but I found less problems with World of Light because it at least knew it was in a Nintendo game with its World Map designs and spirit battles referencing past Nintendo properties, unlike Subspace Emissary, which was just a bunch of fucking literally who's you're battling against with nothing memorable about any of them. Surprisingly, I play a Nintendo crossover because I want to fight and beat up Nintendo characters in Nintendo locations, not OCs in random made-up generic locations that no one cares about.