You can use the End cards alongside your own Outro. Make an Outro designed for the End cards or just put them over a blank screen. The main feature of End Cards is their ability to work on mobile. If you have a lot of Mobile viewers definitely use End Cards.
There is always a advantage to using them and a disadvantage to using them.
One advantage of using end cards is that it really makes it clear as to what video you are showing and you can have a subscribe feature for your own channel.There are many disadvantages such as you can only have them at the end of your video (as the name provides). You also can't really customize them as you are limited to size and where you CAN actually put them. A third disadvantage is that you can't use annotations with them meaning if you used all your card slots there is no possible way of promoting something else as you can't put an annotation there.
I hope this helps and maybe influences your choice of what you will pick
If you are going to use YouTube end cards I'd suggest to make the video cater for it, otherwise it will just look like an annoying pop up. I've seen end cards that have been placed over an important part of a video just before it ends...