Many good points listed above. Bottom line - to answer your question directly - yes this is risky. Exactly how risky, and what you might do about that, depends. You need to consider what the costs would be (in both time/money/hassles/impact on other things) if you miss that flight. How expensive would it be to get on the next flight? Are there other ways of getting there (later flights the same day? train? ferry? etc.)?
In the case of missing a short-haul connection upon arriving in Europe, where there are often cheap options, the risk is somewhat lower. OTOH, on the way back, assuming you were flying the same route, if you were flying from Crete and missed your longhaul flight from Athens to Detroit, the consequences might be much higher - buying a replacement ticket on the spot for that long flight is probably going to be very spendy (probably much more so than going the other way). Of course, on the way home, all you might miss could be a day at work...if you miss an outbound flight, the first part of your trip may collapse as you miss other onward connections.
Personally, if I'm doing something like you describe (and I sometimes do), I always want to have a "Plan B" sketched out (including knowing costs and all my options). Especially on the outbound travel (I have something like this coming up for a trip this summer, where I have a very tight connection planned...for that I have a Plan B and a Plan C just in case). For the return I'm a little less obsessed.
