Detain does hit walkers. Walker abilities are activated abilities and thus are stopped by detain.
Overload is probably the best mechanic to me, because it means that the cards have far more reach than they did before. If you look at something like Cyclonic Rift, you see a bounce spell that can be used much like Into the Roil was in the early game. It's a nifty, if not thrilling, card that control decks can use. Lategame, however, for 6U you can really blow your opponent out if they've established a board presence, meaning that it is like a wincon. Mizzium Mortars is similar in how it transitions from a removal spell to a full-on board wipe of your opponent's creatures.
Populate depends too heavily on other cards, which is, historically, the mark of a mediocre mechanic. Replicating your 8/8 is fantastic, but if you only have a 1/1 or a 3/3, the value drops off steeply. God forbid you have an empty board though, in which case it's useless.
Detain is, IMO, deceptively strong. Archon of the Triumvirate is expensive but Arresting 2 permanents per swing is not something to be underestimated. Blanking your opponent's best creature or freezing their walker (Vraska vindicate?) while also swinging in for some damage is pretty great value. This is also my favorite mechanic from a flavor perspective.
Scavenge is useful, especially Slitherhead, but overall I doubt it'll see much use. Deadbridge Goliath, what seems to be the flagship Scavenge card for now, is, first of all, a vanilla 5/5 for 2GG. It's aggressively costed from a P/T perspective but vanilla creatures are almost always lackluster -- it doesn't even have evasion. Scavenging for 4GG? Yeah, giving +5/+5 is strong, but if you consider what else you could be casting for that kind of mana... it doesn't seem worth it.
Unleash is probably the weakest mechanic in how it's currently manifested. I think Unleash had (still has?) a lot of potential, but they need to push the "If this creature has a +1/+1 counter on it..." stuff, otherwise its a very blank keyword. Adding haste as well would've almost definitely made this broken, but it seems like the cards they've been showing from Rakdos have really been holding back.