f/5.6 --> f/6.3 is 1/3 of a stop. REALLY people? You're going to ***** and moan about 1/3 of a stop? Seriously? OBVIOUSLY, the point of the M is to shrink things down as much as possible. 1/3 of a stop throughout the focal range is a "duh" kind of compromise. As in... DUH... DO IT! (when talking about mirrorless)
Yes, I do take issue with f/6.3 - beecause in practice it likely means T/7.9
Instead of filter thread 52mm, Canon shouild have made it f/5.6 and 55mm or f/4.0 and 58mm filter thread, even if that wozuld have meant 50 grams more or 100 grams more weight and 50 dollars or 100 dollars higher price.
Huh, lets say Canon did make it constant f/4, for about $50 more. Now there is less demand for the EFS 55-250 STM since its slower, bigger and costs about the same. From a marketing point of view that would seem counter productive, I think (I'm no expert!). Sure, it would attract more M users but more people already have a rebel or xxD model camera so you might not wanna irk them by giving the M users the good stuff, would you?
I think by keeping it small and light they've surely compromised but in a good way. I feel like this is what the M was made for. Small, light, compact. You'll not be shooting sports or tracking BIF with it but for a wee trip to the zoo with the kids it'll do just fine!