Regarding Asynchronous ASP.NET Core 2 Web API

i was reading a article on Asynchronous ASP.NET Core 2 Web API from this web sitehttp://www.mithunvp.com/fully-asynchronous-aspnet-core-2-web-api/

the author show with code that how to develop Asynchronous ASP.NET Core 2 Web API.

here is the code snippet

[HttpGet]
        public async Task<IActionResult> GetAll()
        {
            var contactList =  await ContactsRepo.GetAll();
            return Ok(contactList);
        }

public class ContactsRepository : IContactsRepository
    {
        ContactsContext _context;
        public ContactsRepository(ContactsContext context)
        {
            _context = context;
        }

        public async Task Add(Contacts item)
        {
            await _context.Contacts.AddAsync(item);
            await _context.SaveChangesAsync();
        }

        public async Task<IEnumerable<Contacts>> GetAll()
        {
            return await _context.Contacts.ToListAsync();
        }

        //Code removed for brevity
    }

another guy said for the above code

You don’t need async/await in GetAll(), it creates unnecessary empty continuation. However since Task is not covariant you’ll have to change the method’s signature. And that’s good because Task<IEnumerable> is no really good return type.

anyone can share the knowledge that why some one said GetAll(), it creates unnecessary empty continuation ?

also said Task<IEnumerable> is no really good return type

please some one clear the points what other person try to say.

what is wrong in the above code ?

please discuss in details. thanks