I will attempt to answer in general terms as the question touches on pretty fundemental ideas of government.
Firstly, every government contains elements of centralziation and decentralization. Without centralizing power, the government cannot do anything, so it may as well not exist. Without decentralizing power, the government becomes too inflexible and irresponsive, which would quickly lead to paralysis and collapse.
One of the most tentalizing question for people who want to design good government is how do you balance the two necessary elements such that a government can - and this is important - regulate itself to the end of maintaining a balance between centralization and decentralization.
What are the potential drawbacks and dangers of decentralization in
fragile countries?
The case against decentralization is pretty well made since the beginning of civilization. The short story is that states which exercises centralized power are better organized so they conquer and absorb neighbouring states with brute force. Decentralization makes you vulnerable and centralization allows you to not be conquered, that's the gist of it.
It's not a surprise that most civilizations began as autocracy (usually in the form of monarchy) because it is a simpler organizational structure.
The idea that you can have a strong government with centralized power while maintaining a decentralization mechanism for people to choose who sits in the government regularly (i.e. voting in democracy) is a relatively novel concept. And it was arguably only made possible because the idea of "rule of law" coincided with the invention of printing press, which enabled mass replication and distribution of ideas, and most notably codified constitutions. Suddenly the monarch's words do not automatically become reality, they are held accountable by a higher source of authority, namely a document agreed upon by society that everyone should follow at least in theory.
It should not be taken for granted that we live in a world where decentralization feels like water to fish. If authorities violated a law or the constituion, anyone can point to an unified source of law and say they are breaking the rules. It is highly unusual for centralization and decentralization to co-exist in such a way in historical terms.
TLDR: Centralization was the norm for the longest time. Decentralization is a relatively modern development enabled by technological and societal development.
Conversely, is there any literature that argues for a centralized
system in a fragile context?
Centralization is necessary for efficiency. That's kind of true across the board. The question is how much of it is necessary according to your value.
Authoritarian states often make the case that they need maximum centralization not just because efficiency is good. But also that without extreme centralized control, society would fall into chaos. Coincidentally autocrats tend to generate the very same chaos that justify their rule. Food for thought.
On the other hand, democracies can make an argument for centralization as well. For example, it could be argued that the pre-Taliban Afghan government lacked centralized power which led to its collpase, even if it was democratically elected, it lacked the capacity to defend itself so it got conquered and absorbed by a better organized neighbouring power, so to speak. Another example could be the Untied States, where decades of paralysis in its national legislature has produced an extremely powerful judiciary, leading to a situation in which the Supreme Court could remove federal protection for abortion right without democratic recourse through federal legislation.
