There are several issues when using ServiceConfiguration attribute. Yes, it is easier just to add an attribute to your class and forget about its registration in an IoC container. But this is the only benefit you get.

Lifecycle management

When using ServiceConfiguration attribute, the lifecycle of a service is determined before the composition of an object graph. This might lead to weird bugs in the code. For example, some service might be configured as a singleton (with ServiceConfiguration attribute), and it depends on another service.

[ServiceConfiguration(ServiceType = typeof(IService), Lifecycle = ServiceInstanceScope.Singleton)]
public class MyService : IService
{
    private IDependency _dependency;

    public MyService(IDependency dependency)
    {
        _dependency = dependency;
    }

    public void Execute()
    {
        _dependency.Run();
    }
}

At this time, you might not know what lifecycle your dependency has. If it is a singleton, then it is okay. But if the dependency is transient, you might get weird results.

Deferring lifecycle management for later - in the composition root or IoC container configuration, allows you to make these decisions when you see the whole picture - all components you have to compose. It is easier with Poor DI to find lifecycle incompatibilities as a compiler will not allow you even to compile the wrong composition. With IoC containers and their configuration API it is harder, but at least the configuration is in one place.

Single Responsibility Principle (SRP)

As of single responsibility principle, your code should not have more than one reason to change. With ServiceConfiguration attribute you add another reason - lifecycle management and IoC configuration. IoC configuration leaks into your code. If you would like to reuse your class outside of Episerver code, you would need to modify it.

Summary

Unfortunately, Episerver's documentation describes ServiceConfiguration as a first example (Implicit registration of services).

I suggest you always to use "Explicit registration of services." Create an IConfigurableModule module and register your services. You also might want to use StructureMap configuration instead. It would give you more options and more flexibility by splitting different configs into separate registry files.