I can give you my experience of riding the length of the Americas a couple of years ago.
In South America we were on a WhatsApp group with around 70, non Latin American, riders who were in various areas and stages of their journeys.
I rode a 1200gs and my wife was on a 411 Himalayan.
All the riders were on modern machines, there was one rider on an F650 gs (single) and my bike was a 2011 - they were the oldest bikes.
There was a variety of bikes from CRF 250 to 1250 gs nearly all makes we’re represented only Ducati and Moto Guzzi we’re missing.
Make:
It is true that the big Japanese manufacturers, especially Honda, sell bikes around the world BUT they sell different bikes on different continents. One guy had to wait nearly 2 weeks for a chain and sprocket set for his CRF300 Rally because it is not a bike sold in SA.
Other manufacturers sell the same bikes around the world such as BMW, Triumph, Royal Enfield, Benelli and other Chinese manufacturers.
Also, here in the UK (at the moment) deliveries for Indian and Chinese bike parts, that are not available over the counter, are quicker and cheaper than Japanese parts. Whether this translates to other parts of the world I don’t know.
Big v small bikes:
The vast majority of bikes we saw were large adventure bikes particularly BMWs and Triumphs which were ridden by Brazilians and Argentinians - we saw the same in Central America with Mexicans. Most of the roads are tarmac and the gravel roads are not that difficult - obviously things can change in bad weather. You can, of course, find technical routes but it will be a lot of sand or jungle which takes a high level of skill and/or an enduro bike to ride.
The dangerous roads are mostly dangerous because people die from landslides or accidents, if you’ve done a bit of green laning in the Uk or Europe then you will know if you need a lightish bike you can flat foot, or not.
As a ride I don’t think it matters what size bike you take but as a rider it does. I’m big so, for me, my GS is very easy and comfortable to ride at slow speeds but I could have done the trip on a CRF Rally just as well. My wife also has a 1200gs at home but has experience of extreme conditions on a Himmie so chose that bike with confidence.
The only downside of a small bike is the wind on Ruta 40. At times you may have to lie your bike down with you beside it and wait for a lull to ride a bit more then repeat. A few riders on small bikes did not want to risk getting blown off so waited a few days for lighter winds - on a longer trip it’s not a big deal and shouldn’t affect your bike choice.
Reliability: nearly all modern bikes are reliable and should only need basic servicing up to at least 25K miles. You may need to upgrade some bits before the trip on a cheaper bike - decent chain and sprocket, battery etc.
On the WhatsApp group only 1 bike had a true breakdown (out of approx 70 bikes), the automatic gearbox of a Honda AT failed. The time scale of getting a new box sent to him was untenable and even if he could get one it was doubtful he’d find a mechanic to fit it, sadly his journey was over. All other problems were rider made - mainly riding too fast so bits fell off or crashes. Also some bikes were overloaded - we met a guy whose rear shock blew through a combination of overload and riding too fast - took 8 days for a replacement for his KTM 390 but a lot of that was postal problems.
In conclusion I think you should take the bike you want as long as it doesn’t compromise what you want to achieve. We wanted to complete our journey, see as much as we could, we didn’t want to do hardcore riding but didn’t want a difficult road/trail to stop us going where we wanted.
We travelled slow and light and achieved everything we could.
For RTW, I think, it’s hard to beat a 411 Himalayan if you’re not tall. Low seat, very low COG, and that engine just tractors the bike anywhere. It’s ready to go out of the box and is sold around the world. We would often find out of the way places to stay with poor access - on Ometepe island there was no real road access just a horse track and I really struggled with the GS. At places like this we would always take the Himmie, 2 up, when going out for the day - a combined weight of 185kg no problem
However if you want to travel quicker or race around occasionally for fun then a different bike is needed.
My next trip (hopefully) is Stans, Mongolia, ROB, Old Summer Road - so I’ll probably get a 300 Rally. CRF or Voge? Not sure yet but I’m sure they’ll both make the 20K mile trip trouble free.
Happy choosing and enjoy your trip