1-30 is going to be the most difficult grind. Bleys can probably comment to that better than I can, since he still plays, but that will be the only point you'll be soloing. As I understand it, partying doesn't happen much between 1-30 anymore. Even if it does, it will still be considerably slower than leveling in Abyssea. It's absurd how fast exp. gain became once people figured Abyssea out, and you can jump in as a leech starting at level 30. The most obnoxious part will be that beyond level 50, you have to complete a limit break quest every five levels. You only have to do this for your first job, but it will likely be very obnoxious as some of them MUST be completed solo.

The biggest roadblock I see isn't leveling but rather skilling up. I understand that this, too, has become more manageable, but the problem with rocketing from 30-99 in Abyssea parties is that your skill levels (which determine stats such as attack and accuracy for weapons or magic accuracy for spells) are going to be SEVERELY gimped. It will be a grind on its own to keep them up, and something you'll absolutely need to do if you want to be able to complete the story content. Being level 99 will certainly help if you're trying to take down bosses that used to be difficult fights at level 75, but if your great axe skill on warrior is the equivalent of a level 30 warrior's great axe skill because you've just spammed Abyssea, you're going to have a lot of trouble hitting the damn thing reliably even if you've got decent gear at level 99.