I know it's a playtest. All those problems shouldn't have made it out of the committee room. They speak to bad design. And you can not tell if a core mechanic works if what it is powering is obviously broken.
Five guys sitting around a table is never going to accurately determine how much a power or ability steps on the toes of a class.
On paper, going ethereal does seem like it steps on the rogue's ability to open locks. But in play when the warlock is the only one that can pass through the sealed door, the rogue seems pretty useful. When the party encounters a locked chest, the rogue seems pretty useful.
Plus, there's not enough room for every classs to be a unique butterfly with no overlap. The fighter is going to have some fighty overlap with the paladin and ranger (to say nothing of the war domain cleric). So having a potential stealth overlap between the rogue and the warlock is not inherently egregious.
And, in case you hadn't noticesd, playtests tend to start with extremes. Look at the high hp and frequent occurrence of Advantage/Disadvantage in the first playtest. And the low hp now. They're testing the limits to see how far they can go and where the comfortable middle is. That is NOT something that can be done with a committee of five or ten or twenty or even a hundred.
Warlocks should be dark and shadowy and have elements of the rogue. The question is how far can they toe into rogue territory without devaluing the rogue.
Elements of the playtest are not meant to be perfect and are
purposely imperfect to gauge how they work and feel. Because if it was "close enough" it would be ignored and people would focus on minutia.
Furthermore, no single power of a class is a core mechanic. A core mechanic is something like initiative or surprise or advantage or hitting. Testing if saving throws and contests work as intended is a thousand times more important than any one power, or even all the powers in a single class. You can remove a power, you can errata a power, you can even ban a class. But if you effed-up how initiative works the game is in trouble.
Even just for classes, powers aren't that important. Again, it is a 5-minute fix at the most. Because they're independent and separate; you can change a power without having to change anything else in the game.
What we should be testing for the warlock - what its actual core mechanics are - is the pact boons/ favours mechanic.
Does that work? Does it function mechanically or is it too confusing and unwieldy. Do warlocks gain enough powers or too many? Should pact boons and lesser invocations be separate or does tying them to the same recharge work. Is two favours enough or should there be more? Should they be tied to level? That's the basics.
That's what needs to be tested.