It can be problematic in one very particular chain of unfortunate events.
If you ever get a bounty in the system you have legacy fine in, the fine is added to your bounty. If you then die and will have to respawn in a station ruled by the faction who issued the bounty, it'll be added to your rebuy costs.
Whether it is exciting or scary is up to you.
I was so excited to get rid of all those extra transactions. 3.7 mil in bounties, 2 mil in combat bonds, and nearly 4 mil in legacy fines. I've made an executive decision not to pay them or acknowledge their existence from this point forwards.