One could argue that base64 having a shorter output length than its input would weaken any given password, assuming a brute force attack (not a dictionary one).
Adding to this here [1] is a chart that shows how several encodings affect size ratios of utf-8, 16 and 32. Of course that gets into the discussions [2][3] of using utf in passwords and I have no idea how many password schemes support this beyond what I use.
