Ask someone who owns one.....
Top loading dishwashers are fine, long as one does not need to keep much or anything on top of lid when not in use. Am here to tell you that having to shift "counter stuff" off and then back onto the GE Mobile Maid in order to load and unload gets old fast.
As for the rest, top loading dishwashers appealed to certain design sensibilities for same reasons as washing machines. That is no need to stoop or bend.
Think about how one does the washing up; dishes are scrapped then moved into a sink, washed then moved into a rack or dried by hand. With a top loading DW you did the first bit then put things down into the machine instead of having to bend down and load one or two levels of racks.
Yes, there TL dish washers often failed is in the rack design. Most required some sort of planning as to get at bottom level you needed to remove the top rack. Later incarnations of KitchenAid, Whirlpool and General Electric solved this issue by having top racks that raised out of the way when lid was opened; this gave full access to lower level easily.
As for dropping things, yes, it could be an issue. Especially anything sharp and that Plastisol tub liner of GE Mobile Maids. That and or you could risk chipping the porcelain of other machines. Main issue is having to fetch the darn thing out. With the GE MM you have to lift out the entire lower rack to get any anything. This in turn requires removing the upper lift out rack first. Oh and if the lower rack was even partially loaded everything had to come out before you can lift the thing.
My GE Mobile Maid washes the Frigidaire 18" (badged Kenmore) DM design dishwasher under the table, and holds more in the bargain. But the novelty has worn off, and likely soon will simply get shot of the thing. Use the top of any portable dishwasher as "counter space" and having to shift the Maxim convection oven off then back onto the MM is getting on one's nerves.