In Boa Vista both ends of the loop meet. At first I wanted to ride through the Guyanas in a clockwise direction, but for various reasons I decided do ride the Amazonas first, and do the loop in a counterclockwise direction. If coming from the south, you would enter the loop at Belem, and take a boat to Macapa. If continuing south beyond Belem, e.g. going to Ushuaia, you would skip the Boa Vista-Manaus-Belem ride entirely. Similarly when riding north after Boa Vista.
Timing is of the essence. Apart from the 7 to 8 days spent on the river, there are 6 ferry crossings, and some ferries operate only once or twice a day, others on demand. But they don't care much for motorcyclists. So be prepared to wait. Also note that customs in these latitudes usually is closed Saturdays and Sundays. I entered French Guyane at St.-Georges de l'Oiapoque on a Monday, and left Guyana at Lethem again on a Monday, 8 days later. So, it is better to spend more time in Fr. Guyana, or Suriname, than being stuck at an awful place like Oiapoque or Lethem for two days.
Difficulties: The worst section is between Annai and Lethem (140km): mud. (I took me from 6.00 to 11.00 to do it, and I'm no good in mud). Other than that no great difficulties. Regular enduro tires will be adequate (I had Michelin Anakees, not by choice).
Security: No problem, except for Georgetown. Be careful there. It was a great trip. I would do it again, even in retrospect. In regards to safety: There are several signs on the landing site of Oiapoque, Brazil, warning arrivals in French and Portugese, that "AIDS knows no boundaries, wear a condom." I tried. But after wearing the thing continuously for 4 days and nights in that heat, I noticed a significant change in both size and texture. So I went to another shop and bought one 100% cotton, and loosely knitted. I'm almost back to normal now.
One more remark: If you liked my reports, go to the Books section of HU, and find four of my books about riding on all 6 continents, written in a similar style. My nom de plume is Werner Bausenhart, now on a 12,000km commute back home to Canada.