It's just as good as any measurement. Liners are made of different material, are different thicknesses, and some pack out more than others so you'll be in the ballpark.
But you could buy a boot that fits out of the box but after 20 days in them don't fit anymore because of the same reasons above. Plus you have to consider boots won't have a perfect fit and you're better off learning the craft of boot fitting if you're a serious rider.
Sounds like this shell test only pertains to the length of the boot. That is the easiest part to get right and if that was all that matters ordering boot online would be nearly worry free. But you also have toe width, heel width, ankle size, calf size, foot height, and overall foot volume.
Just to be clear... the reason why ski fitters do the shell test is because the the liners than come in ski boots are usually completely made of heat-moldable foam (most snowboard boots only have a little bit of foam around the ankle and toebox area). The inside of the hard plastic shell is irregularly shaped and the un-molded liner is kind of blocky (it is going to squish a lot to match the interior of the hard plastic shell.
Because of this, it is very uncomfortable/painful to wear a ski boot without molding the liners. Of course, you can't mold the liner of the boot until AFTER you've paid for it... which you don't really want to do until you are sure the boot will fit... hence the "shell test".