They are mediocre on their own; the Death factions in general haven't been fleshed out much in AoS (only the Flesh Eaters have gotten a release) and Deathrattle feels the weakness of that. They don't really have all the tools they need to thrive as an army on their own right now. The exception is Tomb Kings. Tomb Kings are more or less OP. However, this isn't due to their foot infantry so if you don't like skeleton chariots then that isn't much of an option.
HOWEVER, AoS matched play is very much still about gauging the power level of lists to be roughly on-par with your opponents. Matched play can be min-maxed to a very large degree, but if your local community is middle-of-the-road you should have no problems running Deathrattle.
In other words: if your opponents are running strong lists then I wouldn't advise Deathrattle as they aren't incredibly competitive, but if your setting is more casual then go for it because Deathrattle are still viable as an army choice.
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