It's only a waste of money if you see this as your only learning, pass the course and think you're now a shit-hot CPO and that's the end of your CPD.
If it's paid for then go do it and get your SIA licence and start work, then look at more advanced training, a driving course, medical, security management, leadership training, surveillance detection training, learn a language...
Look at what your typical Principal does in his or her life and do something to make you appeal to them more than the next guy - SCUBA or Powerboat certification, learn to ride a horse so you can go out with them and keep up etc
Work on your fitness so when the boss goes for his morning run you can stay with him, be alert and not embarrass yourself as the sweaty mess lagging behind.
Look at finance or business courses, initially so you can hold a decent conversation in the car if the boss feels chatty, and understand more about their life, pressures and decisions as you work for them. The more you can integrate and 'fit in' the more valuable you become and the more successful you'll be.
As a CPO you're protecting someone's life, typically someone important, rich or both. They can afford to hire the best and to be fair, it's their life on the line if they choose badly so why wouldn't they pick the best?
So invest in yourself and be the best
The Phoenix course should only be the very start of your journey - it might be a good course, those who warned you off might have a personal grievance behind what they said, or it might be a very average course. Doesn't really matter unless it's the only CP training you ever do as like I said above you're then trading yourself on their reputation, so make sure it's not all you do and it's not an issue.