For a lot of these, they can actually be quite strong if you maximize the use of the cards (playing them for their full benefit for instance:
KC/outpost has some fine synergy in a very thinned deck. Take a simple deck where the only chaining is KC/KC/Outpost/Bishop deck.
Normal turn: I chain the KC's to get an outpost turn (KC/Outpost stays on the board) then play the bishop for 4 VP (crunching a silver) and buy a silver.
Outpost turn: I draw my entire deck KC, Bishop, and silver. I KC the Bishop and I net 4 VP (crunching a silver) and a silver.
Each turn I get 8 VP.
If you can't otherwise chain, two copies of KC can be used to chain so you can play useful actions on both turns. The strongest of these being the unstoppable pin from KC/KC/outpost/masq where your entire deck goes poof the moment I play the combo.
Further having KC around makes outpost turns worthwhile. With good enough odds of hitting KC/most anything decent (lab, grand market, golem, adventurer, etc.) can make a 3 card hand much more worthwhile.
Highway/Bridge/Princess works quite well with trash-for benefit when you either have a lag between acquire (at low prices) and trashing (at high prices) or when you can bring the price of your gained card down to zero (e.g. forging a late game copper into a duchy is quite strong). Gaining a bunch of self replacing cards (pawn, pearl diver, village, wishing well, etc.) for zero cost means that they don't slow down your deck ... but can be fed to the forge whenever you need them to top off a province acquisition.