Alphinaud stands shoulder to shoulder with world leaders often telling them what he thinks they should do, and coming from someone so young and idealistic it can appear that he is both overambitious and naive. The overambition coming from his privileged upbringing, being a prodigy who was inducted into higher education at an exceptionally young age and having a widely respected grandfather. Appearing naive because his youth is paired with being idealistic in stark contrast to the often pessimistic and older world leaders. To someone who doesn't know him well he could appear to be little more than a child who thinks their background gives him more life experience and skill than he actually has and that he thinks being a well educated prodigy gives him the right to be on the same level as those far more experienced and older than he is. In short; at first glance he can look like a well dressed smartypants kid playing at being a diplomat.
Urianger is far older than Alphinaud so from that alone it is easier for him to get respect. He mostly works alone doing research or puzzle solving so he isn't in a position to get criticised as often as someone like Alphinaud who constantly works with highly influential people outside of the Scions. Alphinaud has to constantly prove to people that he isn't just some kid hoping a well tailored outfit will fool them into thinking he has merit.
It was an act of good intentioned arrogance. It wasn't that the idea of the Crystal Braves was bad, it's that Alphinaud didn't take enough care to make sure he recruited people who can be trusted. He was riding the wave of success too much to notice that the Braves were not completely under his control. His idealism blinded him to the fact that he opened the door to people with ulterior motives to gain a position of power, and assumed that the power hungry syndicate that he often disagreed with wouldn't interfere.
The Braves taught Alphinaud that he shouldn't assume that everyone who joins him for his ideals are joining him with the same goals in mind. They taught him he should talk with people to get to the heart of their intentions instead of just talk at them hoping they'll eventually just agree.
His ego was overly inflated due to his background and high intellect but it wasn't tempered with enough failure that older people already gained from life experience. The Braves was a huge failure that taught him important lessons, but they also shattered his ego and he began to question the pursuit of his ideals. However being with Estinien taught him that the tenacious pursuit of the goals from his idealistic views is very powerful when someone knows how heavy the cost of failure truly is.
When the Scions get back to the Source Alisae laments a lack of growth in comparison to the others. Alphinaud, instead of being overconfident and wanting to snatch success like in ARR, simply tells Alisae that their time will come. He knows to just wait because before his inability to have mindful patience made the Braves a disaster.
Alphinaud is my favourite character because he is very flawed. He always had a good heart but he was overconfident. Now his confidence stems from actual experience rather than just raw intellect and admiration for his grandfather. He's still the same height, but the strength of his character grew considerably.