Since they are much more limited in their print run and rarely get restocked, the Pokemon Center ETBs tend to go for more because of their scarcity
Apart from the issue of some cards not having playable substitutes, the other reason I tend to dislike this advice is that part of what makes formats like Commander fun is the diversity that they offer, and this diversity is built, in part, on players being able to access and play with weird, old, nostalgic cards that don't really see play in other formats
The changes that impact MTG Arena are: Cori-Steel Cutter is banned in Pioneer
Another nice point about the new rules is that you can move to the main monster zone by using the first effect
The unearth ability is put on the stack, but the card is not
If you're building a Team Rocket collection, this is one of the centerpieces