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Pros
Pro Machines help automate the tedious tasks
Machines are used to improve yield from ores, such as the macerator. Electro Furnaces don't require any coal to process the ores, which is helpful as well.
Cons
Con Machines can cause servers to lag
Minecraft sorts the world into 'chunks' - 16 x 16 blocks. If a machine is kept running in one of these chunks, it will keep the whole chunk active which can cause servers to lag if there are many people on it (or many machines in different chunks).