Burundi’s post-conflict constitution provides a robust array of formal checks to personal rule. Article 164 mandates a 60-40 Hutu-Tutsi split in National Assembly and 50-50 split in the Senate in order to ensure that the majority Hutu (85%) do not violate the rights of the minority Tutsi (14%). The Batwa (1%) are also guaranteed representation in Parliament through special nomination. Burundi also has a proportional representation (PR) system with a closed list that requires political parties to nominate no more than two thirds of candidates from the same ethnic group. Article 257 of the constitution reinforces the principle of ethnic balance by mandating a 50-50 split in the military. Furthermore, according to Article 300 any amendment to the constitution requires an 80% super-majority in the National Assembly and two thirds of the Senate (this is why Nkurunziza failed in an attempt to amend the constitution in early 2014).
So how did Nkurunziza manage to overcome all these formal institutional checks on his power and engineer a technical third term in office? For answers see here.
Hint: elite consensus on acceptable bounds of political behavior matters a great deal. Looking back, the framers of the Burundian constitution probably should have focused on intra-Hutu balance of power as much as they did on the Hutu-Tutsi balance. Nkurunziza succeeded because not enough Hutu elites (within his own divided party) were willing to punish his blatant contravention of term limits on a questionable technicality. Perhaps they will stand up to him if he tries again in 2020.