Procession is a very interesting concept but with random it's not easy to find a good kingdom for it.
Mountebank, or any other strong attack, makes Procession very viable, even if there's no $6 Action card to upgrade it into. It's important to play Mountebank early and often, and getting it in twice in one turn is huge. OK, you trash your Mountebank, but two Mountebank plays produce $4 in virtual money, so you just buy another one.
Urchin and Mercenary are such complicated concept but I have yet to see a kingdom where they become key.
That is strange, because typically when Urchin is on the board, both me and my opponent rush to get Mercenary first. Trashing two cards is nice, +2 Cards +$2 is great, and it's a Militia too, and you didn't have to spend more than $3 for it. I'd argue that it's too overpowered, and too swingy - the trashing helps you play it more often, and the discard attack especially hurts when your opponent is trying to play their own Mercenary, as they have to keep Mercenary and either two junk cards to trash, or some cantrip/village and gamble on it drawing something they want to trash. Getting that first Mercenary can have such a big impact.
The ruins, while different, does not really create a different game from curses.
They absolutely do; the obvious differences are that Curses are worth negative VP while Ruins are easier to ignore, and if you find yourself with a spare action (especially in a Fishing Village or City deck), you can't play a Curse for some marginal benefit. There's also boards with both Ruins and Curses being given, and having 20 junk cards as ammunition does play a lot differently to having 10.
But there are so many situational differences, a large proportion of boards should see some difference. All the Cornucopia cards that care about variety get a boost, because there are five differently named Ruins. Quite a few counters to Curses - Farming Village, Wandering Minstrel, Golem, and Vagrant, for example - simply don't work at all against Ruins; but also, Curses really hurt Scrying Pool while Ruins hardly matter. If you play Ironmonger you'd rather have a Ruins, but if your opponent plays Tribute you'd rather have a Curse. Cultist vs Vineyard is a recipe for disaster, and in a Gardens slog, you might even want to buy an Abandoned Market, just to help increase your deck size with some extra Coppers.
No lie, I
lost a game once against a Menagerie/Ruins engine.