I've always classed grind achievements as an easy get, they're usually simple and straight forward. There is often no skill involved. (Unless you count determination as one.) It's almost always a case of just sheer repetitiveness, but should we take that into account regarding difficulty? I think maybe it's time I should change my ways and say: Yes. Repetitiveness can make an achievement harder, much harder.
I've done a few grind achievements in my time, most notably all the damn flag achievements in Assassin's Creed. That was truly a soul-sapping experience, so much so that I lost my appetite and couldn't eat for three days. Despite the hardship I endured, I held onto my notion that these were easy achievements, just grinds. Understandably, I think I inserted that thought into my own head to the make the achievements less demoralising while hunting them. Looking back on it, I spent the good part of week doing something that I hated. That made it hard. On the bright side I did lose some weight. (Not that I needed to lose weight.)
Currently I'm going through a massive grind in Gears of War 2 as I am trying to reach Rank 100 for (of course) an achievement. This grind is slow, boring and makes me want to kill children but my need for 'chieve will prevail and I will get that achievement. It's this recent grind that has made me reconsider how I view grinds.
Are grinds difficult? I suppose my conclusion is that if you lack the dedication: Yes, they are, but it's somewhat subjective.