My personal experience has been that Velvia 100 works fine in flat light. Its colors seem a little over the top at sunrise and sunset compared to Velvia 50 but it is still OK. However, at twilight hour (before sunrise or after sunset), Velvia 100 seems to produce a strong magenta color cast which I can‘t get rid of. Velvia 50 is the better option in this case. I use mostly Velvia 50 though. I haven't found anything better than it to capture the subtle hues in the nature under soft light, magic! Even its slow speed works better for me a lot of time as it save me from using a ND to get a slow shutter speed.