Monpods are great but no substitute for a tripod.
Generally they are good for 1 to 2 stops (1/2 to 1/4th the shutter speed) over hand holding. They are particularly good for certain sports as they give support and steady a long focal length lens, but let you switch directions and/or move in a hurry where a Tripod can be in the way.
If you are trying for a very large DoF landscape, then you are probably still going to want a tripod unless you are in fairly bright sun.
The hand hold "rule of thumb" is 1/(effective focal length), but most pros consider this a bare minimum and for a quality shot you would like about 2x faster. A lot depends on the size of your output.
Then we have the "Sunny 16 rule." This says that on a Sunny day at F16 the shutter speed will be 1/ISO. Thus at say ISO 100 on a Sunny day in full sun you would shoot at 1/100th. On a cloudy day or in "bright shade" you can be down 2 stops or more so you might be a 1/25th at ISO100.
Just for example, lets say you use a 20mm lens at F16 in bright shade. The monopod might just get you over the hump in terms of steadyness, but a Tripod would probably still be the better way to go. You could trade off DoF and/or ISO with the monpod, but with the tripod you could get the shot you want (and get it lined up).
For shooting sports I would pick a monopod over a tripod. For shooting landscapes, I would prefer a tripod.
Karl
I am wondering about the benefits of a monopod. Is it a reasonable
substitute for a tripod in landscape and if so, what are the realistic
lens and shutter speed limitations.
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Karl