Personally I consider 30 gallons the bare minimum for salt water even though many have had successful 5/10 gallon salt. Even though some would disagree I also consider under gravel filtration essential with salt.
The problem with undergravel filters in marine tanks is if you have big rocks in the tank. A lot of people want rocks for the fish to hide among and the rocks stop water flowing through the undergravel filter. This means less filtration area and possible dead (anaerobic) pockets under the rocks. Undergravel filters need to be gravel cleaned regularly to keep them clean (just like you clean a power filter) and you can't gravel clean the substrate if it's covered in big rocks. Undergravel filters can be used in marine tanks but not if you plan on having a lot of rocks in the tank. Having an external filter of some sort (canister, trickle, sump, refugium) makes it easier to clean the filter if you have lots of rock in the tank.
With smaller tanks fresh water is probably easier to keep but larger tanks, 100 gallon plus, salt is probably easier. In both cases we are trying to mimic nature which we will never actually achieve. Still, in a smaller tank it is easier to do with fresh water than salt. Seems off but I have found it true that in larger tanks salt is easier.
The bigger the tank the more stable the water quality and the easier it is to keep fish. This is true for fresh, brackish or salt water. Marine tanks have a high pH (8.5) and any ammonia produced in water with this pH is extremely toxic. So the more water there is to dilute any ammonia, the better. However, once the filter has established (by cycling the tank & filter in the same way you do for freshwater) you shouldn't have any issues with ammonia. But larger volumes of water are better for water stability (ammonia, nitrite, nitrate, and everything else that builds up in them). Bigger tanks also give you more options to the fish you can keep.
Fresh water is also easier to treat for illness. Don't know why this is but you can use several antibiotics in a fresh water tank. You can't do this in salt as the antibiotics will kill the entire tank by killing the good bacteria. In salt you pretty much use poisons to cure fish such as copper. It becomes a serious issue of adding enough poison to kill the bad without killing the fish.
Marine fish get treated in the same way as freshwater fish. If antibiotics kill the filter bacteria in a marine tank, they will also kill it in a freshwater tank.
Copper is an old school treatment for marine tanks and was used primarily for whitespot and various other external protozoan parasites. Most of these can be treated with Malachite Green, formaldehyde or other chemicals. Copper doesn't get used much now because a lot of people who have marine tanks have invertebrates in them and copper kills invertebrates.
Having said that, marine fish are not as commonly infected with diseases (compared to freshwater fishes) when you buy them from a shop. This means you are much less likely to need to treat them. And with all fish it's a good idea to quarantine them for a month before adding them to an established display tank to reduce or prevent the introduction of diseases into the main display tank.
