You don't get the caramelisation of the sugars with the slow cooker, which means you don't get the 'roasting' smell or the depth of flavour that goes with it. Also you don't get the risk of burning, the need to pay attention and you dont need the fats .... Like everything in life, there upsides and down ....
In an ideal world and if I'm around and I can fill the oven, then I use that. But ..... if I'm going to to be out for hours and / or it would mean leaving an oven burning fuel without being full or .... I try to work out how to use the slow cooker. Meat is generally better sealed in fat first for a casserole - 'cos then it does the caramelising thing - so I fry it off first before putting it in the slow cooker and ideally soften the onions in the fat too. But sometimes it isn't possible (I can't do it first thing in the morning -heave!) and then I need a marinade to provide the effect (red wine vinegar, olive oil, mustard and worcester sauce overnight as my default for beef).
If you do a boeuf bourginion in the slow cooker for example, fry the bacon till the fat runs, soften the onion and the chopped mushroom stalks in the bacon fat with it and add the garlic for the last few mins. Tip into the slow cooker. Then seal the beef which has been tossed in flour, deglaze the pan with a splash of cooking brandy and add to the cooker. Add black pepper and red wine and water to taste. Bring up to simmer and then set to low where it will simmer for anything from 6 hours to - I think the longest I have left it (unintentionally) was 17 hours. Later add the whole shallots and about an hour before serving the mush caps. Salt just before serving.
But things that need long simmering and no caramelisation such as bacon hock or daal or chick pea curry, even better, as there is no risk of them boiling dry or going to fast.