Like other DNSBLs, SpamCop's SCBL is controversial. Criticism primarily comes from those whose IP address has been listed by SpamCop, resulting in their mail being rejected by servers that use the SCBL service. Third parties that don't send spam are affected when an IP address they use becomes black-listed due to reports that accuse their IP address of being a spam/UBE source. This can happen because their IP address is shared with many other customers of their ISP. It may also be the result of malicious, careless or over-zealous reporting of spam.

Additionally, SpamCop automatically lists IP addresses that send mail to spamtrap email addresses. Since these addresses may be falsely used as a return address on spam messages, backscatter caused by these spam messages (including vacation messages and other auto-replies) can result in an otherwise innocent server being blacklisted if it fails to employ backscatter prevention techiques. One of the unique features of the SCBL, however, is that a listing expires automatically when no spam is reported from that source for 24 hours.

SpamCop notes that "The SCBL is aggressive and often errs on the side of blocking mail. By using the SCBL, you can block a lot of spam, but you also may block or filter wanted email" and suggests using the SCBL as part of a scoring system. Unfortunately many ISPs and IT consultants use the SCBL as a simple screen, often without making it explicit to their clients that valid incoming messages may be (indeed from time to time will almost certainly be) rejected.