But it's always done in a very "I'm saving you all from future heart break matter". Maybe it's just semantics. "We should temper our expectations" is not the same as "I think your expectations are unreasonable".
I think sometimes it's done to avoid some of the heartburn that's come from discussing guys like Miller and McIlrath and even Kreider.
All three of those guys came in with very high expectations and they've had some ups and downs.
Some took a little longer to develop, some took baby steps followed by huge leaps, some have taken two steps forward and one step back, etc.
At various points Kreider has been called a 15 goal scorer and a 35 goal scorer; McIlrath has gone from Shea Weber to waiver fodder to our best defenseman not named McD, etc. etc.
In most cases the truth is usually in the middle. Personally, I tend to keep my mindset on the more conservative side. Early strugges are not a death sentence, and early or sudden success is not an indicator of long-term viability - it usually takes a few seasons for a pattern to develop.
I've seen many kids look lost, only to have them become solid NHL players. I've seen other kids have their best offensive seasons within their first three years in the league, sometimes even their first. With the exception of superstars, it's usually a respectable amount of time before fully knowing a player's niche/production area.