Nmap is fast when the scope is small and straightforward. It crawls when the target range is huge, complex, or locked down. Slow host discovery is the most common complaint. Timeouts stretch. Switches like -T4 or -T5 help, but at the cost of false negatives. Balancing accuracy, speed, and stealth becomes a grind.
Parsing the output is the next problem. Raw Nmap results are dense. XML or grepable output options make automation possible, but each requires custom parsing scripts. For ongoing scans or integrating into CI/CD, this adds maintenance overhead.
Firewall evasion is another source of frustration. Fragmentation, decoys, and idle scans work in theory. In practice, they draw attention or fail against modern filtering. You can spend more time tuning flags than gathering intelligence.