Structural problems within the colonial system are a legitimate concern...
As for terrain though, again, considering the Incas ruled from southern Colombia all the way to the Argentina Pampas from Cuzco with mostly stone age technology, I can't understand how terrain is such an issue. If anything, given the right conditions the Andes would foster such a large state, not hurt it. It might be too bulky if you add in Colombia, Venezuela, Uruguay, and Buenos Aires, but they aren't too cumbersome of additions given that the Latin Americans by this time had guns, horses, and plenty of other things that the Pre-Columbian peoples didn't.
I see your point, but I think there are two problems with this idea of a united South American Spanish-speaking state:
1) The Incas could do what they did because their core territory was Cuzco. From there, they expanded North and South. They never reached Colombia (though they could have had), Venezuela or the pampas. IOTL, Peru was the loyalist center for a lot of reasons. There was so few support for the revolution there around 1800 A.D. that NO revolution took place, and the territory had to be "liberated" from abroad. The revolution started in the perifery of this center: in Venezuela and in "Argentina". From there it expanded very slowly to Perú. It took a lot of time!
By the time revolutionaries reached Perú (more than a decade after the begining of the revolution in 1810) both Venezuela and Argentina had got used to be independent. There was no way they could resign to that, specialy to a central authority located in Perú (the former enemy). And trying to rule from Buenos Aires or Venezuela is extremly hard, as the land route to the other end of the country would imply crossing the Andes several times, and walking trough thousands of miles; and the maritime route would imply traversing potentially hostile Brazilian/Portuguese waters.
2) Even if you somehow have Perú as the center of the revolutionnary process (which imply very old POD) it is difficult for Buenos Aires or Venezuela to accept to be rule from Lima. During most of the colonial history (1500-1776) Buenos Aires was suposedly subjected to the Vicerroy of Lima. But his authority was only nominal in many areas, and he was strongly rejected in Buenos Aires. Even if the Vicerroyalty of River Plate isn't created in 1776, any important change in who's holding authority (or in who legitimises this authority) would be use by Buenos Aires to separe from Lima (which was more hated than Spain).
I think that having 4 or 5 big nations in Latina America (as Metro has said) is possible. Having an über Mexico, an United Provinces of South America, a Gran Colombia (Ecuador, Colombia and Venezuela), a Gran Perú (Peru and Bolivia), and a Gran Argentina (Argentina, Chile, Paraguay and Uruguay*) is possible. But having a united Spanish-speaking state encompassing Venezuela, Colombia, Ecuador, Perú, Bolivia, Paraguay, Chile, Argentina and Uruguay is practically ASBish.
*Stricly Speaking, having Uruguay as part of Argentina is very easy. Having Paraguay inside is more difficult because the Paraguayans had developped a sense of proto-national identity at least by the late XVIII, and having Chile integrated to Argentina is very difficult for historical and geographical reasons
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