Bottom line: this is a very poorly worded spell. Although no doubt others think it is well-worded because it encourages GM rulings in favour of determinate rules.
Yeah... I would even go so far as saying rather than "poorly-worded" or "well-worded"... Barkskin is a classic example of "simply-worded". And when it's come to 5E, WotC's penchant has been to go for that over all other kinds of rule explanations. Stealth is another example of that.
They came to the conclusion that trying to write all rules so they cover every single base any player will ever come up with never actually works. There's
always some corner case that's found or got missed... or even only shows up later on in the game cycle when a new expansion of the rules arrives in a supplement or something. So their way of dealing with it is "Here's a simple rule. If there's a question, the DM can deal with it at the table."
Which on first blush is a fantastic way of handling it... because what's one of the complaints we always hear? Too much errata. Some players don't want to have to print pages of rules clarifications because it's a hassle and it makes them think the rules were poorly designed. But why *is* there errata? Because some other players want all the interlocking rules to be seamless and airtight, which means you have to close up all those corner cases as they appear. They don't want to have to make their own determinations because other players might disagree with them (especially out in the wilds of Public Play), and also they feel like getting the ruleset straight is WotC's job, and not theirs.
I mean heck... even now with Morrus and that other blog collecting all of Mike and Jeremy's tweets about their D&D rulings in one place... half the players want that list to get hammered into a workable format and then posted to the D&D website immediately for use... while the other half find this list to be an anathema because they know that other players are going to trumpet these rules clarifications as though they are the word of god, and refuse to accept the DM's rulings at the table.
So really... there's no actual solution here. And ironically we (and I'm definitely including myself in this) are going absolutely against the philosophy of 5E's design by even having these long arguments about how these spells are supposed to work in the first place. I know I got a bee in my bonnet the couple times folks had posted "Yours is a house-rule, this is the real rule" and I reacted with pretty much a "NUH UH! YOU'RE THE HOUSE RULE!

" When truth be told... I think we're really all supposed to be looking at the system
as a whole as nothing BUT a huge set of house-rules. We're given some basic stuff written down in a book (or several books) and we take them home and hammer them into whatever it is we want them to be. I mean heck... that's the whole purpose of the upcoming DMG's existence. Nothing BUT some more basic stuff to take home and meld into our games.
So screw it. From now on... I'm going to do my best to no longer try and
explain the rules as I see them here on the boards. Instead, I'm going to just state how I'm
interpreting the "simply-worded" information given to me and how that will interact with things in my game. I chose to play 5E... the least I can do is actually go along with their philosophy about the game as well.