Kuznetov actually signed a bridge deal out of his ELC, he signed his third extension after. And it turned out pretty well, he was one of the reasons the Caps won the 17-18 Cup. It only soured in the last few seasons which will happen. Not all 8 years of an 8 year deal will go perfectly.
Monahan got injured. Bad luck
Tkahchuk wanted to sign long term with the Flames, one of the reasons why he demanded a trade was the bridge contract. If he had signed long term, he still could have demanded a trade but couldn't enforce his conditions on a trade. The Flames wouldn't have been limited in that instance since young players don't have NTCs.
Not if you know anything about hockey. The Devils are an analytically driven team, they recognized Hughes talent before hockey fans. Most young franchise players improve during their early 20's locking them up while they're young guarantees you their peak years at a discount.