Are you talking about a chainsaw or a chainsaw mill?
In terms of a hydraulic chainsaw they work well when attached to machines that already have hydro on board, like harvesters, which are basically hydraulic chainsaws on wheels and have been around for a long time. if you can afford one, then good luck to you but they don't serve every purpose. For just cutting a tree down, as a faller I would not want to be encumbered by hydraulic hoses etc. In fact they would be very dangerous. Getting a harvester into some places is also just not possible.
In terms of a chainsaw mill it depends how mobile you want to be and what power sources you already have on hand. If you have to bring along a 4-stroke motor to power a hydro pack to power a chain saw then why bother with a hydro pack - it just robs power from the 4 stroke - just connect the four stroke to the bar or a bandsaw and away you go. If you already have hydro with you on site, like a or a tractor/loader to move stuff, then hydro makes more sense, although if I did that I would want to wait for cutting to stop to use the machine say to move something. For wide cuts a 20 HP four stroke on a Lucas slabber can run a 76" bar without blinking, or run super low rakers on narrower cut.