I have a fully hydraulic machine sitting in my garage. I don't know of any particular disadvantage unless the seals leak. Can you give a more specific question, please?
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"Well, I've wrestled with reality for 35 years, Doctor, and I'm happy to state I finally won out over it." Elwood P. Dowd
Much heavy duty construction equipment is totally hydraulic. The bad side is low efficiency. The good side is the power transmission system is simple, flexible, and self lubricating.