If you're not manually checking the PGP-signed SHASUMs of the software you're downloading for slackware, you're not getting any more security the defective apt software we've been running on debian.

Edit: As pointed out by elosius, verifying SSL certs when you download packages would give you some degree of security (and that's what I often end up having to do on Windows), but unless you have access to signed digests from the package author, you won't get any better security than the broken debian apt system.