It is possible that airworthiness hasn't been a major cause of loss of aircraft for years because we've all got very good at it, whilst piloting ability hasn't changed much for a lot of years. There's some proof of this in the pretty horrendous fatal accident rate within the first few years of microlighting - after pilots licencing came in but before airworthiness.
But, pure structural strength wouldn't be highest on my list of worries because when you compare the low maximum weights with available material gauges and strength, you'd have to try reasonably hard to make a substantially understrength aeroplane that small (so long as it's designed to be non-aerobatic anyhow). My worry would be much more in the direction of very poor stability and controllabity causing eventual loss of control, unrecoverable spins, easy of stall on approach...
Mind you, it's still possible to make an understrength aeroplane, and I'd certainly not discount it as a risk.
G