Personal trainers I've spoken to always say the hardest part is KEEPING their weightloss clients. Most clients are unmotivated, and you need to find their balance of both motivation and support to keep them going, and the difficult thing is some of them can't be pushed or motivated hard enough to get results without feeling like your a hardass, and hating training.
I trained an obese woman lately, and the focus was to find a form of cardio she enjoyed, and a diet she could hold down without it failing in a week and her binge eating again. Seems most people don't have the willpower to maintain a diet now. While discussing diets with her, she actually was intrigued by keto and said she'd give it ago. We tried various different cardio, and some high intensity light weight, it was difficult because everything we tried, she said it hurt. Even walking on the treadmill at 4.5km/hr hurt her knees. Never was able to hear back from her after the first day, and I thought it went pretty well regardless...
Training clients who want to do strength training, powerlifting or bodybuilding in my opinion is easy, but weightloss is almost always difficult.