I am a fan of sumps when possible. Regarding the ATO for planted tanks, it depends on the design of the sump. Evaporation on FW is far less relevant that SW. SW changes salinity with evaporation. FW just increases the concentration of nutrients, normally by a nominal level. The most important thing is making sure your pump doesn't run dry. If your return section is a few gallons, this may be days or a week for it to run dry. For my sump on my 180, I have a 75 gallon sump. The pump is sectioned off with mattenfilter, so it has access to 50 gallons or so. It would take over a month to run dry. The downside would be if the overflow clogs, the display can overfill and flood. With a beananimal style overflow, this is not a concern.
There are some pictures in my build thread if you are interested in some DIY ideas.
Regarding the OG questions
1- Sumps can be as easy or simple to maintain as you want. It is mainly dependent on the design and how much crap you put down there. In FW, this is less of an issue as you wont have a protein skimmer taking space. Most people also do not run many or any reactors or algae scrubbers or....excess equipment. Additionally, stand design will effect ease of maintenance. If you have a sump under the tank and the stand it short, it may be difficult to access the filter. This will be the same issue with a canister, but just something to consider.
2- This is again dependent on the design. For instance, if your drains are above the sump waterline or are not tuned properly (gate valves are the way to go), this will create a ton of surface agitation gassing off co2. Drain lines should sit around 1 inch under the water surface
3-Nanofish will make it into the sump. Depending on design, this may or may not be an issue. If there is a lower flow section, they will make it there and be fine. One major benefit of this is if you have fry, they will make it down there and it is a sanctuary where they will not be eaten. Until I added foam to my canister intake, I would commonly find shrimp in there and occasionally a nano fish fry. They are less likely to live in a canister than a sump.