The last advantage is that Rat Colonys power boost counts all other rats in play, including Verminister and its tokens, Marrow-Gnawers brood, and so on
If Dredge is forced to deal with those, it cannot develop properly
If you do, reveal cards from the top of your library until you reveal a nonlegendary creature card with lesser mana value, put it onto the battlefield, then put the rest on the bottom of your library in a random order
Each dragon has unique abilities and strengths, ranging from massive damage-dealers like Drakuseth, Maw of Flames to card-drawing engines like Dragonlord Ojutai
While it took them a couple of years, we can see from this card from Amonkhet (2017) that they did really find a way to do these cards
Both of these effects are great, and open up a lot of possibilities for resilient decks that can kill opponents out of nowhere