One of my favorite parts of Pioneer is how much new sets have an impact on the format. For Legacy and Modern, there’s such a high barrier to entry in terms of power level that new cards have a hard time breaking through. Pioneer is still small enough that new cards have a high impact on the format. As a result, the format is shaken up more often and is always fresh.
Some of Dominaria United’s most impactful cards are classic reprints that are being introduced into the Pioneer format for the first time.
I love Pioneer, but it’s frustrating to build allied color aggro decks because the mana was just so awful. You were lacking the Seachrome Coast “fastland” cycle and the Adarkar Wastes “painland” cycle, so you just never had enough untapped sources for all your colors. Adarkar Wastes opens up potential for UW tempo, Spirits or flash-style decks. Basically, Spell Queller is a big winner from this land being added to the format. We’ve seen Rakdos decks have success in spite of missing a few dual lands, but adding some Sulfurous Springs will improve the mana for decks like midrange and sacrifice. Karplusan Forest is a big deal because it’s very important for green decks to have lots of sources that enable a turn one mana Elf. This opens up the possibility for Gruul decks to actually curve out consistently
We don’t get access to all of the painlands in this set. We miss out on Underground River and Brushland. Underground River is not a very important card because UB decks usually are more controlling and not interested in paying life. Brushland, much like Karplusan Forest, is extra important because it enables turn one mana Elf. We’ll have to wait until The Brothers’ War to have a strong green/white deck in Pioneer.