I don't know that any of my clients, past or present, like the Prowler. Some like what it does for them, some tolerate it, some hate it. I love it - as a trainer and as a trainee. It's one of my favorite pieces of equipment.
For that reason, I use it a lot, but I also hear a lot of comments about it. There are a few reasons clients give not to do it, or to modify it, that I do find to be valid objections.
Low Handles Make Me Dizzy - Or light headed, or it's hard to breathe, or some variation on that theme. I've found that with some older clients, especially, going from horizontal to vertical can make people light headed. For clients with that issue, I will often forgo the low handled sled pushes. It's just not enough benefit vs. the risk of injury from fainting. In this case, lighter, slower, high handle pushes can fill the whole in the workout.
Toe Injuries - even pushing slowly, with a flat foot, a toe injury - especially a break - makes pushing the Prowler pretty hellish. It's not a good form of exercise if you're compensating to avoid one foot. You can sometimes swap in sled drags instead.
Wrist pain - Sometimes, due to wrist issues, you can't grasp the upright handles without pain. In this case, you can cup the top of the uprights and push. That limits the weight you can push, but a Prowler is pretty effective without a lot of weight.