I love Superior Threads. I've used it exclusively for the last 5 or so years, both piecing and quilting.
If your only concern is the stitches coming apart at the edge of the blocks, keep these facts in mind:
The block edges are encased in the seam allowance.
Only the first 1 or 2 stitches come undone*, so depending on your stitch length, you've got another 3 or 4 stitches to go in the seam allowance, and those stitches are holding fast.
The quilting will provide more stability and keep the seams from coming undone.
*I'm temporarily accepting your premise that they come undone - I've never noticed it happening on anything I've sewn.
HTH