There are many cards which allow for major advantages to be built up early
During the initial months of Pokmon TCG Pocket , ex monsters dominated the competitive landscape
Ghave, Guru of Spores, Slimefoot and Squee, and Nemata, Primeval Warden would benefit greatly from having this new saproling in their lists
This is cool as they can be placed on top of any non-special version of the Pokemon, meaning you can mix and match what base Pokemon you want to use and still get the benefits of the BREAK card without worry of losing out on aspects of the original card
Add to that plenty of ways to put additional +1/+1 counters on it and many smaller synergies throughout the Standard format, and I expect Walking Ballista to be a Standard Staple from the start
This card can attack twice during your battle phase