Did no one bother to go look at the WotC mechanic explanations? It's literally where this card was spoiled.
Most of the rules that govern how transforming double-faced cards apply to the new modal variety. While a modal double-faced card is in your hand, your graveyard, or exile, it has the characteristics of its front face only. So, if you're searching your library for a land card, you can't find a modal double-faced card whose front face isn't a land card. If an effect allows you to cast instant spells from your graveyard, you could cast Valakut Awakening, but you couldn't play Valakut Stoneforge.
It is, but as noted, having one in your "opening hand" 100% of the time is a big difference.
Also, while Power Artifact is cheaper and blue, this guy is not prone to 2 for 1 removal on your artifact. If your Monolith gets nuked in response, you don't have to find 2 more pieces to assemble the combo again.
Also now you can run 8 Monolith and 8 reducers if you want, making the combo very easy to assemble without tutors.
So this is a very reliable way to have infinite mana with Grim Monolith.
The casting cost is as bit annoying, but it shouldn't be that bad to get out consistently.
The deckbuilding restriction isn't too bad as you can still have mostly instants and sorceries. It does however prevent you from playing Young Pyro and Mentor, but you can play all the Planeswalker you want.
As a side note, I know they needed to give it some kind of activated ability, but the no block thing is just so out of place on there.