Since you asked for limitations, I am not mentioning the advantages here.
Some of the limitations that I can think of for the firefly luciferase are
1) The less signal intensity compared to the fluorescent proteins
2) Since the signal is less, you will need to have a longer exposure time to get the images.
3) Since the exposure is longer, having higher temporal resolution is a limitation (but I think in your case you will not need a very high time resolution to monitor metastases)
However there are now brighter versions of luciferases from different sources that you can try if you are going to start fresh. It is just because the new versions are much brighter compared to their firefly or renilla counterparts. And also some papers (link below) that say about the usage of luciferase in monitoring tumour metastases.
Article Fluorophore-NanoLuc BRET Reporters Enable Sensitive In Vivo ...
Article NanoLuc Reporter for Dual Luciferase Imaging in Living Animals