Ultimate Brave Burst information is in the datamine. Its just not in the units list. Data follows:
Vargas
Name: イグニードヴァイザー Description: 敵全体に超強力な炎属性２４連ダメージ＆2ターン攻撃力を超絶低下
Effect: 24 hit AoE, reduce all enemies' ATK for 2 turns
Stats: Damage modifier +1000%, proc chance 100%, ATK reduction 80%/2 turns
Selena
Name: シロキセカイ Description: 敵全体に超強力な水属性２５連ダメージ＆３ターン味方全体を全回復し、さらに通常攻撃ＨＩＴ数をアップ
Effect: 25 hit AoE, regenerate all allies to full HP for 3 turns, increase Hit count for 3 turns
Stats: Damage modifier +1000%, 99999 regen/3 turns, increased hit count/3 turns (some values in increased hit proc are unknown)
Lance
Name: リブル＝セブン Description: 敵全体に超強力な樹属性２１連ダメージ＆３ターンの間、味方全体に防御力に応じて攻撃力がアップする効果を付与
Effect: 21 hit AoE, converts 100% of all Allies' DEF to ATK for 3 turns
Stats: Damage modifier +1000%, 100% DEF added to ATK/3 turns
Eze
Name: 全てを破壊してやるぜ！ Description: 敵全体に超強力な雷属性３連ダメージ＆３ターン味方全体、スパーク時のダメージを超絶アップ
Effect: 3 hit AoE and spark buff for 3 turns for all allies
Stats: Damage modifier +1000%, spark dmg + 120%/3 turns
Atro
Name: ティル・ナ・ノーグ Description: 敵全体に超強力な光属性２３連ダメージ＆３ターン味方全体のＢＢゲージが超絶増加
Effect: 23 hit AoE, regenerate BB gauge for 3 turns for all allies
Stats: Damage modifier +1000%, BB regen +50/3 turns
Magress
Name: ブラックエンド Description: 敵全体に超強力な闇属性２２連ダメージ＆３ターン味方全体の防御大アップ＆１ターンの間、受けるダメージを１にする
Effect: 22 hit AoE, increase ally DEF for 3 turns, reduce all damage taken to 1 for 1 turn
Stats: Damage modifier +1000%, DEF +150%/3turns, 100% Mitigation/1 turn
UBB have 10 levels available, however all levels are identical, so presumably you don't actually need to level them (probably placeholders to fill in the database and prevent errors). They also have associated BC costs, but I have no idea how that's relevant at the moment so I haven't included them.
