If you want to limit the risk of wasting your money, then you should try adding the sample pack to your next order as its pretty inexpensive
The Huel flavour boosts are formulated so that they balance the taste against not adding too many calories to the mix. Store bought alternatives may or may not do the same. Some, such as powdered peanut butter or unsweetened cocoa/cacao can do this but others mean a larger increase in calories.
Are the Flavour Boosts worth the money? If you like them then yes they are as they offer a good balance of taste vs. calorie increase. Tastes vary wildly between people but you can read this thread for people’s opinions.