The Euro influx into the NBA is significantly weighted towards bigger players, ie 6'9 and above. How often are European guards in general drafted in the first round vs European bigs? And a large portion of that is 7-footers.  I think the genre of Euro player that should set off alarm bells is "jump shooting big" or "perimeter oriented 7-footer". These players usually lack toughness, and were designated scorers in Euroleague, with other players handling the responsibilities for rebounding, defending etc. When they are asked to take on those responsibilities for the first time at the NBA stage, several issues come up.

If there is something I've learned from the Raptors Euro-Experience, it is that trying to teach a perimeter oriented big to improve the hustle areas of their game is a risk that I'm not comfortable taking. That's something that comes from inside (see Evans, Reggie)

 Another issue is due to a lack toughness, since others handled the less glamorous duties. Once they reach the NBA,  they sometimes end up having to play one position down from their true position, and can't cope when they are at a disadvantage across the board as an athlete. Bargnani can handle playing center if one was rating his physical tools including athleticism. It's a lifetime of being coddled as a big SF that keeps him from holding his own in that area. Contrary to what some believe, interior defense and rebounding are present in the European game. It's just that Bargnani had nothing to do with those areas in Europe either. 

The fact Dirk Nowitzki kept on improving year to year is a testament to what a special person he is as a basketball player. I'd say the work he put  into his game (and progressive improvements) given his physical tools is in the same class as Nash. That kind of dedications is rare in anyone, North American or European. His ability to succeed as a perimeter-oriented big says MUCH more about Nowitzki then about the ability of any given jumpshooting big to transition to the NBA. He has only served to establish a ridiculously high best case for these players, one which is based largely on factors which were unique to Dirk and not generalizable at all. Part of what makes Ray Allen great is his unparalleled dedication to get up shots hours before anyone else is at the stadium. Just having a pretty shooting form isn't enough. Part of what makes Steve Nash so great is his great hand-eye coordination, and perhaps benefiting from his soccer background like Hakeem did with his footwork. Being ~ 6'2 and having great range on his three doesn't make Jimmer Fredette the next Nash. 

Drafting a European big that PLAYS like a big, that's a risk I'm comfortable taking. Maybe, it is something in the water, but Eastern European countries (Slovenia, Lithuania etc) produce more then their fare share of NBA sized bigman. Thus, as long as a European teenage bigman plays tough, I would not discredit their level of competition in areas like toughness, size, physical strength. It's likely they are being tested in these areas as much if not more then in say... mid-major competition, since they are often competing against men instead of solely college players.

If you have concerns about athleticism, I agree that there is no substitute in terms of athletic tests, then facing North American competition. However, those athletic advantages are lessened (ie vertical, quickness) as the player  bigger. There are few bigmen in general that have great verticals or quickness. Their size works against them in that regard. Given the scarcity of bigs that even reach that size, bigmen are in for a shock regardless of where they competed prior to the NBA. Players like Kevin Garnett, Dwight Howard, Ben Wallace, they are very few and far between. Thus, you just need to deal with it and make adjustments when facing that calibre of athlete on both sides of the ball once you reach the NBA. A bigman that can't keep up with European competition athletically is a red flag. However, if they possess a certain base level, I would not count them out because their head doesn't reach the rim when they dunk. It is players in this scenario that benefit from being scouting at U-17, U-18, U-19 international competitions, where they are more likely to come across players that are at their level or beyond in terms of athleticism.

There are also strengths on the European side of the pond in terms of practice time and skills development. I believe the limit in terms of NCAA practice time is 20 hours a week. And they can't start practicing until a certain date (in september?) unless they are starting their pre-season with exhibitions out of the country. That is the reason why we have so many US schools travel to Canada, to play CIAU schools, get a jump on their training in the process. European teams have no limit on their practice time, and train for most of the year.

Big men go bust at a high rate regardless of where they're from. It's the product of what happens when so many kids in school can keep progressing solely because they're so much bigger and stronger then the competition. In comparison, a six-foot guard has a much harder road to travel on his way to elite level basketball.To typecast Europeans draftees in general as substandard, without looking a bit deeper at whether there are relevant subtypes can lead to teams missing out on great talent for what could to be superficial reasons.