I'm the one who posted on WW, and stumbled across this thread. A couple of quick things:
-I hadn't even assembled the bike when someone asked about the Di2 port for the FD, which I hadn't seen. When I finally built the bike I discovered it, and updated my post on WW. I have no intentions of running Shimano on this bike so I hadn't really even thought about it. Plus, I was busy test fitting a ton of tires on different wheels...
-Regarding the tire size, some folks here are missing the point. The choke point is not the width, it's clearance at the tread cap. "I tried 44's or 45's and had x clearance" doesn't really mean anything... The seat stays taper in--pretty severely--towards the top. As a result, the first 50c tire I tried, a 50c RX Pro, was actually hitting the frame at the tread cap on one side on a 32i rim. It measured over 50mm, so I fit it to a 25i rim. The clearance was still no where near adequate. Clearance at the sidewall is absolutely fine, but as tires get wider, they get taller, this choke point moves up the stay, and the sides of the tread cap are perilously close to the stays. It's a simple geometry problem. It's also why extrapolating clearance on a 44c tire and thinking "oh, a 50 will fit" is just not at all useful.
There are some 50c tires that will fit, not question; however, many of the most commonly used 50's will not. They are passing that info along, which is great, and I know they are doing some of their own testing as well. Note that my bike is a size M, and since the other sizes are using different molds, it's possible this may be different on other sizes. I can only speak to my frame.
One last thing to note, these are all static measurements. I have yet to encounter a gravel tire without some runout at the tread cap. Some are worse than others, but 1mm seems to be the minimum. That means that 2.75mm of clearance becomes 1.75mm--or less, depending on the tire. The runout is always going to be significantly worse at the tread cap than the sidewall, exacerbating the problem.
A first they were honestly a bit dismissive (not the CS rep, but their engineering team) until I pushed back, and then they dug a bit deeper and realized I wasn't just making things up. They revised their recommendations, which is great. They are also doing some more testing, and I recommended they test a few specific tires to possibly come up with a list of tires they can recommend for those looking for a 50c tire in the rear. All of this is regarding clearance on the rear; the front meets the originally published spec of 50mm with 6mm of clearance, and while it's a bit tighter at the crown I found it adequate with both a Dub and RX Pro in a 50 on a 32i rim, so I have no issues there.
Big picture, the bike will work great for me, which is why I'll deal with not being able to use my favorite rear tire, the RX Pro in a 50. I have another gravel bike with more clearance, but this bike will see dual duty when I'm traveling, and I'm just as likely to do a crit race on it as a gravel race, so it's a tradeoff work making. For those who want more clearance though and for those looking at fitting a MTB tire or 55 gravel tire, they should look elsewhere. I think it's just good for people to understand the limitations, because they are real, and as I mentioned to them I think it's pretty key that they set customer expectations up front. It's just better for everyone involved.
As far as the comment about it being an aero gravel bike: well, so is the new Factor, Crux 5, Scott Addict, XMCS, Tavelo Grow, new Grail, etc.... The fact is that the aero gravel market right now seems to prioritize big tire clearance, so in that sense this bike is going to be hamstrung a bit by the tire clearance issue.
