I have replaced absolutely everything below the frame. These things are basically junk.
Tires - because I don't want cheap Chinese junk.
Wheels - to fit real tires.
Brakes - because the drum armatures were severely worn.
Axles - because they were bent from new.
Springs - because they broke.
Spring bushings - because the cheap nylon was spent.
Shackles, bolts, nuts - because they were failing.
Welded frame - because it was cracked in 4 places.
Stabilizer jacks - because they were assembled wrong.
Look at Northwood Mfg. for a company that makes a good frame and uses suspension parts that are not at the very minimum that is legal. Look at the NCC (net carrying capacity) for similar size trailers.
The "I never had a problem" crowd are living at the edge.