IMO, nothing wrong with using the EZ-lube system, when following the axle manufacturers instructions.
Used to be, likely still applies, not a good idea to mix different types/brands of grease. You have no idea what grease was used by dealer. A seasoned mechanic would know, just using the taste test! :)
After the first time of hand packing the bearings myself, I have used the EZs, with good results. A couple squirts would do little, as you'd need to pump until fresh grease shows up at the front bearing.
Jerry