Dummy.cs

public static class Dummy
{
    public static void Run()
    {
        Debug.Log("Run!");
        Observable.Timer(TimeSpan.FromSeconds(1))
            .Subscribe(x =>
            {
                if (Random.Range(0, 2) == 0) Success?.Invoke();
                else Failed?.Invoke("エラーメッセージ");
            });
    }

    // コメント

    public static event Action Success;
    public static event Action<string> Failed;
}

DummyRunWrap.cs

public class Result
{
    public bool Success { get; set; }
    public string ErrorMessage { get; set; }
}

public async UniTask<Result> DummyRunWrap()
{
    var resultTask = Observable.Amb(
        Observable.FromEvent(x => Dummy.Success += x, x => Dummy.Success -= x)
            .Select(x => new Result { Success = true, Error = null }),
        Observable.FromEvent<string>(x => Dummy.Failed += x, x => Dummy.Failed -= x)
            .Select(x => new Result { Success = false, Error = "message" })
    ).First().ToUniTask();

    Dummy.Run();
    return await resultTask;
}

Sample1.cs

Dummy.Success += () => Debug.Log("成功!");
Dummy.Failed += error => Debug.Log($"失敗... {error}");

Dummy.Run();

Sample2.cs

var result = await DummyRunWrap();
if (result.Success) Debug.Log("成功!");
else Debug.Log($"失敗... {result.ErrorMessage}");