I am not a coach any longer, but was coaching middle school when I started playing HD. We ran a flex offense and the varsity coach wanted the JV and middle school teams to all run the same offense. I understand the concept of the Flex better as a result. I think it's a much more complex offense than most people realize. What I learned is that the Flex is really a hybrid of a Triangle and Motion offense using some of the same skills of both. (Probably too complex for the middle school team I was coaching, which made our wins less at that level but probably helped them once they got to Varsity since their "IQ" was higher after running it for several years already.
Motion is probably the most difficult offense to recruit, since all players will play the point position equally once the ball is at half court, so all players need ball handling and passing skills. Balance is key for a motion as everyone plays a large part of the offense. From watching teams play, I tend to believe that a low post game is most underutilized aspect of this offense since you are constantly in motion and very rarely stop long enough to post up against a defender. Perimeter is needed for all players since you are either spotting up for a long jump shot or getting a lay up 90% of the time in this offense.
Triangle is probably the easiest offense to recruit, but has it's own challenges since big men need to be good passers and you need a strong low post player and a strong perimeter shooter to be effective. Triangle is better when you want to have 2 or 3 dominant scorers and then you want the rest of your players to be pretty good passers so they can get the ball to those scorers. Athleticism is usually pretty important for those scorers because the triangle will open up lanes for the scorers to drive the lane.
Flex however works more of a combination. It's kind of like a stand still motion. Everyone rotates around the wheel, however the ball pretty much stays at the top of the key or on the wing with passing, so ball handling isn't as necessary for everyone. However the flex opens up the lane for inside scoring, but it also tries to get the defense to key on the inside shot and leave a wing player open for a jump shot. Balanced scoring works best, and I always try to use a PG, SF, SF, PF, PF/C lineup to give versatility with perimeter shooting and low post game with as many players as possible. Where the motion has everyone facing the basket, the Flex the post players are usually facing away from the basket looking for a pass inside, but the guards also can have opportunities to drive the lane more than a Triangle. I love the flex, but (in real life) it requires a higher IQ to perform better and you need more balanced scorers with inside/outside game.
How this equivocates to the HD sim, I have no idea..... but that's the basics of each offense that I learned while coaching.