If his deck is full of cheap removal, then the chance to try and get in damage after you've cleared that Sable off means it's probably right to hold back, whereas if his deck is full of 3/2s then you'd want to lean toward trading off
Eternatus has endured across generations by being more than just a legendary Pokmon
Lastly, Petilil and Lilligant are a great secondary Pokemon choice to synergize with Venusaur, due to Lilligant's Leaf Supply move, which can attach additional Energies onto benched Pokemon each turn, helping to get Venusaur set up and ready to use Giant Bloom as soon as possible
Theyre not over-designed, based on the theme of the Arcana
Remembered fondly by some for the Deck Master concept while also derided for stopping Battle City in its tracks
The other downside is that to transform to Optimus Prime, Autobot Leader, Optimus Prime, Hero needs to die, which is not easy to do even as a sacrificial blocker in combat because of his 8 Toughness