I've played quite a lot of Adventures (2-4 player games) and I think the Travellers are a bit overrated.
The first thing to realize is that they're costed like Chapel. By and large, you want to buy one copy of a Traveller and you want it in your opening buy. So, like Chapel, the Travellers are often going to cost you three coins and an opening buy. (Caveat: I have bought multiple Peasants for multiple Disciples. I think this is a reasonable approach on some boards.)
Then there is the opportunity cost. Page and Peasant have terrible on-play effects. Herbalist and Pearl Diver are rightly at the bottom of the rankings, and Peasant and Page are strictly worse. They're even worse than that, because you need to play them on turns 3 or 4 (Herbalist and Pearl Diver are better in the mid-game) - and you probably payed three coins for them. Soldier is a poor card. How realistic is it to have another attack in play on turns 5-7, when you play it? You would need an action splitter or Urchin. So, it's terminal silver and an attack that probably does nothing (it might do something in multiplayer games). Treasure Hunter is a really powerful Silver gainer (especially in multiplayer), but it increases decksize, which is detrimental to Travellers. So, it isn't until turn ~8-12 that you begin to get positive value out of your opening three-coin purchase.
Then there's the speed to get to the good stuff. You need to see your Traveller five times before you hit the jackpot. Without support, but also without ever missing your shuffle, you should expect to see Champion/Teacher around turn 15-17. And Teacher does nothing until the following turn. There are a lot of kingdoms where that is just too slow. If it ever misses your shuffle (very likely without support), you won't see Champion/Teacher until turn 18 at the earliest. Forget about it! Thankfully, the Peasant line has a fantastic penultimate card in Disciple. When I buy Peasant, I often have Disciple as a goal. Exchanging for Teacher is an option if everything is going well. Hero is not a great consolation prize (better with Bank or Platinum).
With the right support, the Travellers can be fantastic, for sure. Deck accelerators like Ironmonger and Spice Merchant synergize very well. Alternate VP lengthens the game, which makes it very likely that Champion/Teacher will get great value. All said, the Travellers are great cards. But so are Stonemason, Hamlet, Squire, Courtyard, and Fools Gold.