For me the frame is stiff enough and the handling is very good. I am 90kg and ride pretty hard and I dont notice too much flex.

I really enjoy the compliance of the frame. In fact so much that I no longer like at all to ride my old aluminium road bike (Canyon Ultimate AL) that is way too stiff and unfomfortable.

Thanks to everyone for sharing their experiences. Made it much easier to plan my build.

Got mine ready now. Its my daily driver / utility bike. Had no issues at all with the frame. Seems of good quality, and plenty robust enough for my rather demanding use case (carrying a lot of gear in wintery conditions). As others have said, the frame is rather heavy, i.e. a lot of fibre has been used. This should make it heavy duty.

The ride quality is excellent with DT Swiss GR 1600 wheels and 42mm Conti top contact winter II tires. Hard to say how much of it is due to the frame but all the small chatter on our northern European icy roads is pretty much eliminated. Exactly what I wished for. Frame seems stiff enough too. Will only know this for sure with summer tires fitted.

Some details / experiences:
-   In the front I have Shimano road drivetrain (Ultegra) and in the rear Shimano mountain drivetrain (Deore 11 speed).
-   The chainring bolts of the Ultegra 46-36 crankset (R8000) touched the frame so had to use a small spacer on the drive side (the one that came with the FSA T47 bottom bracket)
-   In the rear I have the 'Wheels Manufacturing Dropout 653 Derailleur Hanger #653 DM Mountain UDH'
-   David from Spcycle told me the frame bosses are rated for a rack with a maximum load of 5kg. Since I need to haul groceries for the family with this bike I put on my old Old Man Mountain rack, rated for over 30kg load when using it with a Robert Axle Project thru axle as the rack's mounting point. Their specific thru axle I am using is 'Axle for BOB Trailers 175/183mm - M12 x 1.0mm'. It comes with the necessary spacers needed for attaching it to the frame. Attachment through the derailleur hanger is not ideal but good enough. UPDATE 20 Dec: Using the Tailfin SRAM UDH Adaptor you can achieve a proper attachement of the rack to the thru axle - this is strongly recommended.
-   I have the SKS Bluemels 55 basic mudguards. There are functional mounting points for these on the seatstays and on the fork legs but none in the bottom bracket area (I used a zip tie to attach it to one of the screws under the down tube) and none in the rear of the fork, above the tire (I drilled a hole into the mudguard to use the mounting point directly under the fork crown)
-    With the mudguards the 44mm wide and 44mm high tire (measured) is the largest one that can be fitted in the rear. A wider one would be ok but not a higher one. The limiting factor is the distance between the tire and the seat tube. In the front a 50mm wide and high tire would fit easily even with the mudguard.
-   I have Shimano BR-UR300 brake calipers front and rear. My 160mm rotors were too small for both (in the 55mm offset position in the front) so had upgrade to 180mm ones

Very happy with the frame and build overall.