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Afghan political analyst Mohammad Isaqzadeh agreed the unity government was unlikely to be effective in addressing the huge challenges facing Afghanistan, from fighting the Taliban to raising tax revenue for the cash-strapped government and improving the lives of Afghans mired in poverty.
"Because it is a government divided, there will be competition between the two camps," Isaqzadeh said.
Despite the doubts, there was a sense of relief that a damaging political deadlock had been broken, at least for now.
The deal, more than three months after the disputed run-off election, ensures that a new government will be in place when NATO combat troops withdraw at the end of the year, leaving the fight against Taliban insurgents to Afghan forces.
It also means that Kabul can finalize a Bilateral Security Agreement with Washington that will allow about 10,000 U.S. troops to stay on, crucial to the battle against an increasingly bold Islamist militant movement.
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Spokesmen for both candidates brushed aside the concerns.
"We are stronger together," Ghani aide Tahir Zahir said.
Abdullah spokesman Mujib Rahimi said the new administration "will be effective and efficient."
The United States pressed hard for the deal to end an impasse and reduce the risk of violence between supporters of the two candidates, who draw their support from different ethnic groups that fought a civil war in the 1990s.
A senior U.S. official said he believed that Ghani and Abdullah, both pro-Western technocrats who had similar platforms in the election campaign, would be able to work together.
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Some diplomats in Kabul were less confident, saying the new government faced similar challenges to the system left in place by the Soviets after they withdrew from Afghanistan in 1989.
That involved a rotating presidency under which leaders were changed every six months.
The post-Soviet government, which like the U.S.-brokered deal was formed with the promise of foreign aid money, fell apart soon after the collapse of the Soviet Union and overseas funding dried up.
Civil war broke out between different factions of the anti-Soviet "mujahideen" militias before the strict Islamist Taliban seized power in 1996.
Thomas Ruttig, of the Afghanistan Analysts Network, said other countries' experience with unity governments suggested they were only temporary fixes for entrenched power struggles.
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Those that do work tend to be in countries with strong civic institutions such as parliaments and courts, and Afghanistans' are seen as weak.
"Afghanistan has a deal-making culture, but very often deals paper over the real problems," Ruttig said.
Among those problems is Afghanistan's complex network of ethnic power brokers, ex-warlords and other influential figures in both the Ghani and Abdullah camps who claim to command the loyalty of members of the security forces.
One hallmark of Karzai's administration was to largely balance those interests, but this year's election crisis showed how fragile that balance was.
A key concern in post-election negotiations was the threat by Atta Mohammad Noor, the powerful governor of Balkh province and an Abdullah backer, to launch widespread unrest if his candidate was not declared the winner.
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"It's not only about the two leaders," Ruttig said. "It's also about their followers and backers - some of whom have shown they are more likely than their candidates to play with fire and the threat of violence."
Some Afghans worry that the competing interests of powers that seek influence in their country - including Iran, Pakistan and India - may play into how the U.S.-brokered deal works out.
"Afghanistan's enemies and neighboring countries ... are waiting to see if this agreement brings a crisis," said Kabul parliamentarian Qurban Ali Erfani.