if you are using a wead eater. get one that has a "STRAIGHT" shaft, and no bend in it. and you can use a metal blade brush cutter. the curved shaft weadeaters, you end up breaking the cable fairly quickly.
i have used a old push mower. (metal box) vs the cheap plastic things they have now a days.
then you have wead eater on wheels, or rather instead of a push lawn mower, with a blade, the front is open and you have a thick heavy string. this work on good amount of stuff, exception once you get into thicker weeds and larger diameter weeds. then you have problem on some of them. were if you tilt the stupid thing to one side or forward or backward, and all the oil in the engine goes to one side, and you start blowing out smoke and oil out of the exhaust. cheaper ones are better suited for around the house and shed. were you have long straight lines to work with. if you have lots of trees and things to trim around weed eater on wheels can take more time.
then you have rotatory cutter 3pt attachment for tractor. it is a requirement to have a FEL and bucket on. when doing steep bank sides, not to mention counter balance for rotatory cutter. the FEL can help pull yourself back up a hill. when backing down to pond edge.
then you have self propelled rotatory cutters. other words push lawn mower on steroids. the wheels are normally 1 to 1.5 feet in diameter. and you have the rotatory cutter out in front. this can make things easy, but boy are they a bear when it comes to try and turn them on a steep hill and your footing is not always the best. a good amount of time at least on hills around lake / pond here. you have to drive kinda up at an angle. to drive straight across the hill.
then you have self propelled sickle bar mowers. just like the rotatory cutter above. but instead of a blade spinning, it is a sickle bar mower. sadly the sickle bar does not stick out to one side, but is centered on the machine.
then you have side mounted mowers on booms that attach to center of tractor between the wheels, or perhaps placing a hdyrualic motor on a small size bush hog, and placing it on a backhoe (swapping out the backhoe bucket)
and you also have sickle mowers that attach either in center of tractor between wheels, or on 3pt hitch. as others said, they do move up and down some. but that angle you place them. can be a problem.
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overall....
a 3pt rotatory cutter, plus FEL on tractor. while slow and scary. works good to reach over edge of a bank. just get one small enough that your 3PT hitch lift can easily lift it. and having a TNT (top and tilt) just having the top link hydraulic link. can make things much much easier. you get into situations were tires sink, and 3pt hitch begins to bottom out on the dirt. and having ability to lift that rotatory cutter higher, so it does not catch on the bank as you try to drive back up the hill really helps out a good amount. 4WD or MFWD on tractor can also help get up and down them steep hills leading to shore edge.
using a straight shaft weed eater. with a metal blade vs string. can work fairly well when doing the edge of bank, right at the water level. granted you really have to watch your footing. and it takes a toll on the body, but it allows you to reach off the edge of bank a little bit. and work the uneven bank edge.
for steeper hills, a self propelled rotatory cutter or self propelled sickle bar. can be a descent option. they really do not reach over edge of the bank edge. but when dealing with nasty steep sloops. they do work better than weaving a hand held weedeater with metal blade back and forth over an acre plus of hills around a pond. were you have bushes / trees to go around were a 3pt hitch rototry cutter can not get to easily when i say work better. i mean you can actually move at end of day. vs hand held weed eater.