There's a wide variety of player skill. Someone who's intimately familiar with Mids, and spends a bunch of time tweaking builds and IO'ing their characters to within an inch of their lives is going to have a much easier experience than someone who either doesn't know how to do that, doesn't care to do that, or doesn't have the in-game funds to implement their build.
I think the question is a little broad. The experience on a team is going to be different from the experience solo, and both will differ based on level. So when we're talking difficulty, are we talking difficulty in a team? Solo? Lower level? Higher level? Sure, a team of level 50 IO'd incarnates will blow through content; though I chalk this up to the game closing early before the original devs were able to implement more challenging late-game content.
There's also a wide variety of difficulty for various archetypes. In my experience, brutes are pretty much easy mode from start to finish. Blasters can be amazing at higher levels with enough IOs, but might struggle until then. Support archetypes are a great power multipliers on a team, yet depending on the power sets can struggle solo.
And, another complicating factor is that people play differently. If you actively avoid risk by power leveling to level 50, twink out a character with IOs, and only then tackle lower level content and task forces, of course it's going to be easier. Frostfire is a very different experience at native level 10 with training origin enhancements than it is with the extra powers, power slots, and whatever set bonuses you might have on an exemped level 50. Or take the Sutter task force, which starts at level 20: pre-SOs. Regardless of whether you know what you're doing on that one (and it can be confusing to legit newbies), it's going to be harder on DO's with fewer powers and power slots.
Anyway, I answered, "just right" mainly because, in my opinion, there's quite a bit of nuance to that question and someone can find a play style that will be just about right for them.