I remember seeing someone's miniature REO Speedwagon abandoned in a factory yard, with it's BMX rear wheels folded through 90 degrees. I think wide hubs and thick spokes are the best way to go. That or at least 125cc wire wheels from a motorcycle.
True, it's also worthwhile to note the differences in rim loading, a trike or quad's wheels subject to a lot more lateral force, flex can be the primary contributing factor in failure, even impact damage is usually not as severe if the rim was true and hit on center - rims "taco" most often if hit off center & bent over, they're not nearly as strong sidewise lol
An interesting difference in the strengths of aluminum & steel rims is while steel tends to fare better in impact, it is more prone to lateral flex then the typical double-walled aluminum, a far more rigid structure for it's size & weight then a typical "heavy duty" steel rim.
An advantage of steel is it can take some amount of flexing & warping & regain it's shape whereas aluminum less likely to distort - but once it does it's usually over. Thing is the more the lateral deflection, the weaker the wheel structure becomes & with trikes & quads this side load is almost constant, every even slight turn & there's loading from the side, unlike with a leaner. Of course as is true with all wheels, the larger the wheel the greater the lateral leverage - so smaller is stronger in this case.
Motorcycle steel rims are a solution, overcoming the flex issue with sheer mass - however there's a penalty in weight & cost. I personally prefer as short a spoke length as I can manage (smaller OD/larger hub and/or flange) rather than as thick a spoke & wider double wall aluminum rims. This reduces weight and flex & in my trike's case - unsprung weight, which aids speed stability over bumps.
As with anything, there's more then one solution & no problem quite the same - but I've been quite pleased with the three Alex DX32 double wall aluminum rims (2 20" & 1 26") on my taddy, despite the 14g spokes & additional weight of a hybrid system (about 120lbs dry) I've not had any detectable flex or deflection, I've never even needed to retrue my wheelset;
