It can greatly depend on the player and their style. But I don't really like the way the question is phrased, I'd rather see it read, "Is Omaha Hi/Lo more challenging than Hold 'em?". The answer there is a resounding YES!
There are a greater variety avenues for you and your opponents to travel down and reading the board is all-important. Likewise, getting a handle on your opponents playing style can really dictate your profitability. I like both games equally and I'm rarely board with either, but if I get distracted while playing Hold 'em I generally don't get lost where as if I get distracted while playing Omaha I can easily find myself screwed before I even realize it.
Omaha, particularly H/L is a lot more challenging than hold'em, but in a different way. I get not only bored with hold'em, but frustrated by the very nature of it, which skills seems more based on
bluffing, semi-bluffing, aggression, and position. Not that Omaha doesn't involves elements of each. But I wind up folding more hands that I feel are probably winners, and can feel myself getting tilted when some idiot is constantly raising with nothing. I know, my job to catch him, nuts or not, but it's not enjoyable sometimes. I run into less of that in Omaha of any type.
And of course, NL, PL, FL all play into betting strategies in Omaha. But all things being equal, you're going to see the river more often in Omaha, particularly H/L. And BECAUSE of this, I also agree that distractions are bad news. You do yourself a great disservice to not pay attention to the many hands you might get to see people calling and raising with every step of the way pre & post-flop.