Ohhh i hard disagree there. I think Charlemagne is among the best leaders in the game (of those revealed so far) and one of the most fun for builders actually.
- Free happiness from building adjacency, which helps mitigating penalties from overexpansion AND helps deal with the crises, which often force you to slot in minus happiness policy cards
- Free expensive units that you don't have to build, which help you defend against the AI, which are also stronger than normal if you can chain up Celebrations.
- Also since you're getting 2 Cav for free, you won't need to build or buy as many units, which means you can save your gold, or spend it on getting more buildings
- Automatically unlocks the hyper defensive and expansionistic Norman Civ
- Scientific attibute point which guarantees a small trickle of science, allow you to be beeline to unlock Cav before your first Celebration trigger.
One of the biggest threats to being a peaceful builder is being invaded. He completely mitigates that problem with only upsides. I would currently put him near the top of the leaderboard in terms of strength, alongside Ben Franklin (who I think is the actual strongest leader in the game of those revealed), both Ashoka's and Lafayette.