In constructed, these all seem kinda bad, or at best niche.
Volcanic Drake -- Compare to Flesheating Ghoul. Based off arena experience with the Ghoul, it's not too hard to pump that to a 4/3 on the turn you play it, but doing better is rare. So I see 4 mana as the typical case for this guy, and while 6/4 for 4 mana is a little above par, it's arguably worse than Yeti's 4/5. As a pseudo-4-drop, it competes with Twilight Drake and Hungry Dragon in dragon synergy decks, which is tough. The only deck archetype I can maybe see playing this is Freeze Giants Mage, as a follow-up to a board clear.
Drakonid Crusher -- I can't see this being played if Fel Reaver isn't. Fel Reaver is great in a vacuum (unconditional 8/8 for 5 mana) but bad in a meta flooded with BGH. This guy shares the same BGH vulnerability, but the 9/9 stats are conditional. He's also 6 mana instead of 5, which is a big deal because he's competing against the likes of Sylvanas, Piloted Sky Golem, and (in Hunter) Savannah Highmane. Maybe he'll see play in dragon synergy decks simply by being a dragon, but I wouldn't bet on it.
Nefarian -- The quality of a random spell seems pretty low. It's possible that this might be played, but it seems totally outclassed by Ysera, which has superior 4/12 stats instead of 8/8, threatens to keep drawing cards if she isn't removed (which is a big deal because removing 4/12 is hard), and draws undercosted cards. The only advantage Nefarian has is giving you two cards when he's played instead of just one, which seems outweighted by the worse card quality. If the meta becomes super duper slow, maybe he could be run in addition to Ysera.