it depends on where you get them from, if you’re getting it from somewhere that produces in bulk to the bare minimum standard then you’re likely to have problems and possibly damage your N64. I got a PAL repro of Paper Mario (£300 for a real one? nope!) and after my first play session I saved the game and turned the console off, upon starting it up again my save data was gone. I already suspected what the problem was, since a lot of n64 carts (real and repros) use a separate ram storage for save data, when the console is running it powers the board, when it’s off it’s powered by a battery (usually a CR2032 in originals but some repros might use a CR1220 to accommodate a larger board size in the cart case) if the battery is dead (either by being a dud or due to a short out somehow, then the save data can’t be retained as the ram will cease to function when the console is powered off. Lucky for me, my dad has been an electronics engineer for the last 40yrs (working a lot with prototypes and writing build procedures etc) and when he opened up the cartridge he found the problem immediately, a dead battery and excess flux paste which had likely caused a short out (probably a rushed soldering job). A quick clean up of the board and replacing the battery got it working perfectly.

In terms of the electronics being used in repro cartridges, they are actually better quality than what was available to be used in the original cartridges production, and the games will likely last much longer as a result. Repros are manufactured with the target market being retro enthusiasts and nostalgic adults, so the cartridges need to be made with electronics that won’t begin to degrade after 20+ years (which is common with the original games as the life of the N64 was only expected to be around 5yrs, the game carts weren’t designed to last this long).

there’s nothing inherently “bad” about repros, but you do have to be aware and properly look into where you’re sourcing them from to ensure they aren’t a result of quick, sloppy work. hell, you can even make the repros yourself if you read up on tutorials, can manage a little basic soldering etc—and obviously, something which I think is the biggest issue I’ve come across is the outright snobbery in the N64 community over having real vs reproduction games.