One issue being that the slot limit method is a hard cap to the amount of bionics you can install, and those aren’t as much fun. You can’t actually make a bionic monster of any kind and the player is heavily limited, plus, there’s no way to build in such a way that you can get more bionics. It’s just a flat “the player can install X slots worth of bionics”.
If you do the weight system, or something similar to the weight system, or the slot system with the weight system once all free slots are filled, then there’s only a soft limit in terms of what the player is willing to put up with. If you want to make a bionic monster with every bionic in the game then you CAN, BUT you’ll suffer some pretty substantial consequences. It’s similar to min-maxing in balance terms. You don’t really want to get rid of it entirely, but you don’t want it to be standard play, and you definitely want there to be downsides.