I think by itself it's one of the worst diets I've heard of. Hamsters and gerbils are omnivores that naturally feed on a variety of seeds, grains, nuts, etc... Along with some plant matter and insects. To say this diet is good because it has no seeds shows a complete lack of research into the nutritional needs and natural diet of these animals. They are not guinea pigs. They are extremely different from guinea pigs and do not graze on grass. They forage through a wide variety of foods mostly consisting of the reproductive parts of plants not the leaves and stems. A properly mixed seed and grain based diet is what a gerbil or hamster needs not a grass based pellet, cube, or similar. On top of that it's 1 diet for 2 different species. That's never a good sign. While hamsters and gerbils are quite similar and you can easily modify the diet of one to match the other you can't expect them both to eat the exact same thing. Hamsters need a higher energy diet with more calories, fats, and proteins while gerbils will get quite fat on a good hamster diet.
However the Healthy Handfuls does make a good addition to a home mix. I included some in my new mix along with sunflower seeds, oats, parakeet, and parrot seed mixes. Using the bird seed mixes means I get more variety without having to buy each type of seed and nut in bulk to add to my own mix. Variety is the best way to have a healthy gerbil or hamster along with not overfeeding them so they have to eat the entire mix. Only give them what they will eat in a day and make sure they aren't hiding the bits they don't like somewhere in the cage.
Now if your not mixing your own the best food I've found is Supreme pet foods' Gerri gerbil or Harry Hamster. If you can only find a quality hamster mix for your gerbil add some seeds like the parakeet seed mix to lower the overall fat and protein ratio. Or the reverse add more nuts and sunflower seeds to a gerbil mix for a hamster but usually the hamster food is easier to find. For example mazuri hamster and gerbil is designed more towards a hamster and lacks variety, it's made of plain cubes, but for about 6months I used it as a base and then added parakeet mix to it for the gerbils.