Super Mario Crossover is basically Super Mario Bros. with a selection of NES characters available in addition to the Italian Stallion; they all control more or less like they did in their respective games, and they bring the classic tracks. Some cleverness is imbued in how they respond to fire-flowers and mushrooms, and the rest of the game is SMB down to the tiles, except you can interact with these tiles a bit differently. Subtle things like jumping height and the prerequisites for breaking bricks will jump out at you, carefully built gauntlets will unfurl as hackable, the relevance of four-decimal floating point values will pop out as the underlying soul of the game design you thought you knew growing up.
Robot Unicorn Attack is a well-executed platform-racer in the Adult Swim suite of games, which increasingly seem targeted at sexually frustrated 12-year-olds who, in this title, may relish the dark irony of a cute, rainbow-laden unicorn, girled out to the max, being subject to random explosions on collision with a tear-dropped, severed head remaining. They'd say "yeah! Fuck rainbows and unicorns!" before reloading for a higher score and absorbing the faux-80s song that slowly gives them the gay.
What's interesting about these games in contrast is how they serve as an example of cultural pastiche extending into game mechanics. The obvious fan service of the first game and testosterone baiting of the second is on the surface, but how do the mechanics serve these aesthetics? In classic academic style, I'll answer my own question while struggling not to pontificate. The prior scrambles your training for each game to provide a sort-of dissonance against your memories of what you're already played, for instance the infuriating Belmont leap, with its rigid physics, is partially cured by a double-jump but still feels wrong in the horizontal Mario levels designed for an inertia-laden plumber. Robot Unicorn, on the other hand, tries to create as smooth a flow as possible while bombarding you with aesthetics that may seem threatening to the target demographic, so you're compelled by the gameplay while at the same time repelled by the aesthetics, the ludic equivalent of being seduced by a transvestite. Contrasting these two suggests a sort of converse relationship that is both interesting as a design study and also as a historical example of what contrast old and hyper-extended genres can produce after all this time and mutation.
They're also pretty fun so enjoy them while eating a bag of Milanos and clutching a Teenage Mutant Ninja Turtle action figure.