I am not sure why Frontier's bag enforcement is news.

I took Frontier on at least 7 legs last year; there was bag enforcement on every single flight. It is 100% clear from my personal experience that there is no "slack" in their policy whatsoever, in a general enforcement sense.

A few observations:
1) If you have a typical or even small shell carry-on case and don't have a carry-on paid for - which is glaringly obvious because they board all paid-for carry-on passengers first - you are going to get nailed. A possible hack is paying for "boarding first" but I never tried it.
2) If you have a backpack or a soft bag that is bulging: stuffed full and rigidly extended to full size: you are going to get nailed.
3) If you are carrying 2 items that are even remotely close to the personal bag size limit, you can largely expect to get nailed. My wife got away with it once; she didn't the other time but was able to combine the Popeye's chicken bag with her purse (that was literally all that was available in that Atlanta terminal on short notice). The only other person I saw get away with it was a mother with child and another woman. The Frontier people didn't quibble about the 3 bags hanging off the wheeled child carrier plus the purses etc. Good on them.
4) The checking does get a little less strict once the first wave of fools get nailed. At a certain point, any given Frontier flight can get enough of a line of people paying for their policy violation bag that it jeapordizes the departure schedule. This is not saying that egregiously violation bags are going to blithely breeze through, this is saying that borderline will much less likely be checked.
5) The personal bag size - for those who haven't done this - is very much NOT normal carry-on size. It is a purse or a computer bag. On every single trip, I have combined my computer bag with some snivel gear into a medium sized soft shoulder bag. This bag is not stuffed full and is clearly partially empty albeit very heavy (15, 20 lbs?). I have only been stopped by the policy checkers once - it was after they let the wife through with 2 items - but I fit comfortably in the check-personal-item square just like I had planned even though they made me rotate to ensure both x and y axes would fit in (the soft bag is like 1.5x to 2x the side dimensions of the square box).
6) My only slightly negative experience was not getting a $1000 voucher for volunteering on my 1st return flight on Frontier because I had not checked in online. Grrrr.

For my personal flights, I had no significant concerns about either getting flagged or passing the check. But for my literal free flight to Atlanta (Frontier has had several instances of 100% free flights this year so far i.e. just pay taxes) - I paid for a single checked bag because I was traveling with the wife. To put this in perspective: The actual flight tickets were $130 round trip for both of us. The checked bag more than doubled the final price to $265 but it was still a smoking deal. And it was fine going to Atlanta on our Atlanta/Charleston/Hilton Head adventure.
However, the wife bought A LOT of stuff. Extending the 40 lb weight limit on the bag would be $100. Getting a 2nd bag would be $100. So we did the classic "wear 2 hoodies" and filled the carry-ons because Frontier notably doesn't restrict carry-ons by weight. This return flight to SF is when I got flagged for the only time out of 7 for personal item check.

Overall, I am very pleased with Frontier's pricing, overall experience and value proposition. It isn't for everyone but they are transparently clear about what you get (and don't get).

First of all - this might be just me but Frontier flights are FAST. Every plane I was on that went more than 1000 miles - the plane was doing 550 mph vs. the usual 500. SF to Atlanta flight time was like 4 hours east, 4.5 hours west. Denver to Florida was 4 hours.

Second: my first 5 flights were extremely fast boarding, very little overhead use etc. I thought this was the rule and even foolishly told my wife - and was then immediately disproven by the SF-Atlanta and return flights. But then again, those were the 100% free flight days so perhaps this is an exception to the rule.

Third: Frontier is CHEAP. $19 one way flights, SF to Vegas. <$400 flights from SF to Florida in the summer (vs. $800 for all other choices on Orbitz). The profit has to come from somewhere else...

Fourth: Frontier seats are amazingly small - they are clearly squeezing multiple extra rows on each plane. The only complaint I have is that the fold-down serving tray is so small and metallic-y that on a red-eye back home - the guy behind me kept falling asleep while leaning forward against the back of my chair, and slipping his head/arms/body/whatever in such a way that it deployed the tray with a loud metallic bang. I was understanding the first 3 times but then I snapped and told him to please stop doing that. And I know it was the metal tray because I then proceeded to do the same thing to the passenger in front of me...once.

So the Frontier value proposition is: cheap, fast, cramped.

I did have one experience of a significant delay; there was a bizarre mixup between what the airport displays were showing re gate change and new departure vs. what Frontier was telling me via text and what Frontier was showing on their web site - and the final result was none were correct although the Frontier web site was closest, but 1 out of 7 is a lot better than experiences with say, Allegiant.

YMMV.