Being old enough to qualify for a bus pass has taught me a few things (and I did train and qualify as an engineer anyway). I have made a few trailers over the years but in the past there only seemed to be one brand of suspension unit available for the DIYer. These have suspension arms made from rectangular-hollow-steel which makes a wonderful rust trap for there is no good way to de-rust and paint the insides of the arms. Even with a shot-blasting pistol there will be blind spots. Acid bath and hot dip galvanising is not possible either as the heat will destroy the rubber. Basically the cheap design is a form of built in obsolescence. The better suspension units, for example Avonride have solid arms so are unlikely to surprise one by collapsing! The budget units are OK when new (but isn't everything?). The problem is avoiding arm failure. If the hollow arms are filled with sand and cement using a strong mix such as 1 part of Portland cement to 2 parts of sand one is in with a chance of durability, especially if the trailer is only used on land. Hollow arm trailers used to launch boats are very iffy but there are still a couple of firms making this type.
Avonride and Bramber both make excellent suspension units and basically you get what you pay for. It was ever thus! (Meredith & Eyre suspension units are also excellent - most of the road compressors in the UK run on M&E suspensions)
Regarding trailer wheels, the smaller the wheel the faster it rotates and the bearing life gets reduced. Heat build up in the tyres can also be a problem as the tyre bends back and forth more often with small wheels. Note that trailers used to have a 50mph maximum speed limit but the limit is now 60mph and a modern turbo diesel can tow at this speed all day. On long runs check the tyres for heating. More air and less speed can save a lot of trouble. Trailers using Land Rover wheels are fantastic as the bearings and tyres run nice and cool even with heavy loads. Size matters!