I ran the playtest scenario yesterday, for a party of the pre-gen fighter, cleric and rogue plus a custom human wizard, and our experience was similar to yours.
The party never felt in any real danger, and with the skill/ability check DCs set as they are, any time multiple characters could make a check the party never failed.
We did have a good time nevertheless, especially in the Wormwrithings quest. I enjoyed running Miglin as a somewhat obfuscating character with a fascination for 'interesting' rock formations. Favourite part was as they were setting off:
Wizard: Wait, are there still purple worms in there now?
Miglin: I don't know, I'm not there now.
Wizard: Well, when did you last see a worm there?
Miglin: About six months ago.
Wizard: Oh, that doesn't sound so bad. Let's go.
(Ten minutes later, as they're about to enter the Wormwrithings)
Miglin: Ahh, it'll be interesting being back here again. Haven't been down here in about six months, since I saw that purple worm.
The random rolling in the tunnels was a little swingy. I think we'd been through every other encounter at least twice before I finally rolled for a singing crystal formation, and then they got two sets of three crystals in a row.
The kobold dragonshields were the only monster that posed a significant problem to them, and I'm not sure I was running them properly - it was hard to tell whether their protection ability covered themselves as well as their adjacent allies. I ran it the former way first time round, and the Disadvantage caused them to shrug off attacks for two full rounds. It seemed very powerful either way - I felt that it ought to have required a reaction to use.
The blind ogre encounter was just embarrassing. The wizard moved up on first initiative and Thunderwaved the ogre back behind the kobolds, after which they ran full-tilt into the middle of the cavern to get out of its weapon range, and were picked off by the rest of the party. The ogre never got close to anyone.
By the time they'd collected all the crystals and were heading back, I described the tunnels as echoing with the occasional cry of kobold war bands - not in anger as they pursued the party, but in terror as they desperately fled out of their path.
The wizard found the Sleep spell rather swingy with its hit point limit. It was very impressive first time out, taking down six 3hp kobolds on a roll of 18 points, but when he tried to use it upon a band of four 11hp orcs, it was frustrating to roll 21, just short of taking down a second orc.
All in all, we had a good time, the rules played well, but XP values and monsters' effectiveness definitely need some tweaking.