They can be worth it if you plan to eat those specific things. Assuming you get a 139 HKD coupon and actually eat a 139 HKD meal, you can essentially get a 40 HKD frozen treat for free. But if you eat a cheap meal at Starliner Diner and grab a cheap frozen lollipop or forget to get your frozen lollipop/popcorn, you can come out behind. So ask yourself if you actually want quick service for lunch and the frozen lollipop/popcorn. I think some people who buy these vouchers end up forcing themselves to get those snacks when they'd rather have other snacks or are fine without snacking.
Someone can correct me if I'm wrong but I think you have to show your code to someone after entering the park to claim your vouchers which can sometimes have lines and be an inconvenience. Just give the CM your voucher when you would normally pay to use them.
It really depends. As @gelatoni fan mentioned, you can definitely save money if you're fine with their specific meal offerings. The offerings are limited and you're only allowed to use the vouchers at certain restaurants/locations. For example, at one food court location, all of the lunch meals were priced at HK$119 or less, so you would never get full value there. At the Royal Banquet Hall, there were more dinner options, but also some items that did not qualify that we would have liked to try. We did skip other restaurants/food options only because of the vouchers, which we would have loved to stopped for otherwise. And there were a couple of the voucher-accepted food locations that were closed (disappointing!), requiring us to walk much further while already very hungry. If saving money is your primary goal, then the vouchers are fine. We probably wouldn't buy them again, as we want more freedom to eat at any restaurant nearby when we're hungry, and the flexibility to sample anything that looks delectable at the moment. And the difference in food costs probably isn't significant for 1 or 2 days of HKDL.
Regarding use, we bought online and they scanned the codes at the entry gate, and gave us our park tickets and meal voucher tickets at the same time. At the restaurants, they just scanned the meal vouchers at the cashier. Very easy and no issues whatsoever.
Hope you have a great trip!
ETA - Added some photos (hopefully you can see them)