New player- how does death guard play like?

I’m a new player to 40K, but I’ve played killteam for 2 years. I am attracted to death guard because they look like they are a blast to paint and I love the way the models look. I know that they have the contagion aura and they are slow and hard to kill. To me it doesn’t seem like a worthy trade for their inability to make it to objectives for the first half of the game. Again I don’t really know much about the game so please tell me anything/everything you can about the army!