1 pointWhen scaling anything up, you will potentially artifacts. The severity of which depend on the direction (up or down) as well as how much and what you are scaling by. As you are doing an affine scale, you won't suffer any effects due to change in aspect ratio. Are all those images the same size BTW?
1 pointHow large is the texture? If the width is large enough then some devices might have problems with shader precision. You can test if this is the case by changing from webgl rendering to canvas rendering temporarily. If it works properly on canvas then having a smaller texture or splitting the current one into smalle chunks might help.