SLC means that phase information is included besides intensities and your data is still in slant geometry.
For offset-tracking GEC data could be sufficient as long as the resampling didn’t destroy all of the texture information which is needed for the cross-correlation.
Please also see here:
I’ll have to try if i experience similar issues with my ALOS data.
Edit: The ones (ALOS FineBeam Dual, SLC) I retrieved from ESA as Third-Party-mission data work fine in SNAP.