Oh man! My massive, massive, edit didn't save yesterday.
First off, If you've used a negative rise stem full on XC race bike then you'll feel somewhat at home here.
I changed "downsides" to considerations, as for some they will be problematic but you can make adjustments to compensate and solve these but some people will face issues which can't always be assessed right away or riding.
1. The biggest issue that some will face is that as seat tube angles go beyond 75 degrees or so, there's a chance that some will experience knee issues from increased forward pressure and the reduction of the angle of your legs. It puts more force in to your knees so certain people will experience knee pain. We are going to see more of this over time, as it might not become apparent for a while. The 'free' solution is to move your seat to the back of the rails, or you can use an offset seat post. Both of these things reduce the steep seat angle and maybe defeat the whole point of the steep seat tube angle. Instead you should swap to shorter cranks if you think this might be an issue or it becomes an issue over time. If you're building frame up, I dont think there's any disadvantage to use 5mm shorter cranks than you would on a traditional bike.
2. you need a longer stroke dropper post to get the same feel as an older style bike. Just a note.
3. Forward hand pressure/arm/shoulder pressure could be a problem for riders that prefer a relaxed fit. This new school geo puts similar forward leans to negative rise XC bikes, but with slight riser bars and stems rather than flat bars and upside down stems. So some less athletic riders will get tired faster or experience hand/arm/shoulder discomfort. This can be remedied with riser bars, shorter stems, riser stems (who uses these on real bikes though?), and in an extreme example, going to a frame size down (which people do to themselves all the time, for better or worse).
As you can see, they can all be dealt with, but if you've never touched a 'new school' bike then they're worth keeping in mind. Most young/apathetic people wont have any issues whatsoever, but if you've had injuries in the past or similar you could run in to problems.
