github.com/spf13/viper

Viper handles the messy reality of configuration management extremely well - merging environment variables, config files, command-line flags, and remote sources with a clear precedence order. The initial setup is straightforward for basic use cases: `viper.SetConfigName()`, `viper.AddConfigPath()`, and `viper.ReadInConfig()` gets you running quickly. Binding to structs with `viper.Unmarshal()` works smoothly once you understand the mapstructure tags.

The pain points emerge when debugging why a value isn't loading as expected. Error messages like "While parsing config: yaml: line 5: did not find expected key" don't tell you which file or what Viper was trying to do. When values aren't binding correctly, you'll find yourself adding `viper.Debug()` and dumping `viper.AllSettings()` to figure out what's happening. The documentation covers features comprehensively but lacks troubleshooting guides for common issues like nested config binding or why AutomaticEnv() isn't matching your variable names.

Community support is decent - GitHub issues get responses, though sometimes slowly. Stack Overflow has good coverage of common patterns. Once you've worked through the initial gotchas, day-to-day usage becomes productive and reliable.

Viper excels at configuration flexibility—multiple formats (JSON, YAML, TOML, env vars), live reload, remote configs. The API is intuitive for basic usage: Set(), Get(), Unmarshal() work as expected. However, day-to-day use reveals concerning security patterns that require careful handling.

The library doesn't sanitize or validate inputs by default. When unmarshaling user-controlled config files, you're responsible for all validation. Error messages often leak file paths and structure details that can expose deployment information. Remote config support (etcd, Consul) uses whatever TLS defaults the underlying client provides, requiring manual hardening. There's no built-in secrets management—sensitive values sit in plain text unless you implement your own encryption layer.

Dependency-wise, Viper pulls in a heavy tree including fsnotify, yaml parsers, and remote KV clients even if you only need basic file reading. CVE response has been slow historically. For production systems handling sensitive data, you'll spend significant time wrapping Viper with validation layers, secret management integration, and hardened error handling. It's feature-rich but definitely not secure-by-default.

Viper is the de facto standard for configuration in Go, and for good reason. It handles multiple config formats (JSON, YAML, TOML, ENV), automatic environment variable binding, and live config reloading with minimal boilerplate. The ability to unmarshal directly into structs with `Unmarshal()` makes it easy to work with strongly-typed config objects. The hierarchical key access with `Get()` works well for simple cases.

The main friction point is the global singleton pattern that dominates the examples. While you can create instances with `viper.New()`, most documentation pushes you toward `viper.Get()` and friends, which makes testing harder and creates implicit dependencies. The `AutomaticEnv()` feature is convenient but can lead to mysterious behavior when environment variables unexpectedly override config files. Error messages when keys are missing are minimal - you often just get zero values with no indication something went wrong.

Despite these issues, Viper significantly reduces config management boilerplate. Once you establish patterns (use instances, explicit binding), it's reliable and feature-rich enough to handle complex scenarios like multi-environment configs and remote config stores.