Are non bijective sboxes weaker (on feistel networks)?
No.

It is one of the main features of a Feistel network that both decryption and encryption just need to evaluate the S-box in the same direction. It could potentially be a weakness if you used invertible functions in the S-boxes: First, you are limited to a much smaller set of functions to choose from. And then you might not be able to find a function which has the other important properties, which are actually needed for security, e.g. non-linearity, diffusion and confusion, a balanced output, etc.