I don't think instance_eval is too nasty. The toughest "good" codebase I've worked in was difficult because it used method_missing magic everywhere, which built tons of methods whose existence you had to just infer, based on configuration stored in a database. So most method calls could not be "command clicked" or whatever to jump to their definition, because none were ever defined.

The two are not mutually exclusive. On many occasions I've used C# to define domain-specific environments in which snippets of code, typically expressions, are compiled and evaluated at runtime, "extending the language" by evaluating expressions in the scope of domain-specific objects and/or defining extension methods on simple types (e.g., defining "Cabinet" and "Title" properties on the object and a "Matches" extension method on System.String so I can write 'Cabinet.EndsWith("_P") || Title.Matches("pay(roll|check)", IgnoreCase)').