Yes it is true.Once the screws on them get loosed,the lenses will suffer uneven stress and can break easily,so you need to be much careful with them.Handle them with care and tighten the screws regularly in case of getting loosed.
As the screws are placed directly on the lenses to hold the whole pair of rimless glasses,the lenses can gradually disintegrate on the side where the screws are situated and finally the lenses can drop and break down.So you need to be much careful with the rimless glasses.
Although rimless glasses are very fragile and earier to break than other glasses,you can still use them for a long time if you take good care of them. So remember to keep them in its case When you don't use them, which will give better protection for your rimless glasses.