50 wt is generally a thinner thread than 28 wt. of the same ply. But you can't judge the strength of the thread by the wt. In fact thread may lose its strength with age, depending on how it is stored. We can break any thread with our hands, but some break more easily than others. If it breaks easily... and that is a relative term... then you wouldn't want to use it. Chances are that your 28 wt thread is fine, but you might want to switch to a thinner thread for piecing because it takes up less room on the bobbin (meaning fewer bobbin changes) and also uses less room in the seam, so that it may be easier to get the correct seam allowance.