Probably almost any method is effective at learning guitar, as long as it includes the key factor - time spent practicing.
(1) the songs were already in my head,
(2) Sting would have two or three cool hooks per song, and this is the important part,
(3) the hooks would played over and over during the song. That meant I could play the song all the way through and get to practice each riff 10 times or more with just a single needle lift.
A prime example: Demolition Man (https://www.youtube.com/watch?v=vf7To6vdg7A)
There are a few pedagogical points here to keep in mind:
first, there are local maxima in terms of learning something like guitar where you get bad habits and the only way to progress is to undo them.
Also, different ways of learning have different values in terms of what goals you're aiming towards and very importantly what kind of practice will keep you motivated in a sustainable way. Sometimes, taking shortcuts in some ways means you might slow down your growth rate but you'll have better overall growth because you'll keep at it for longer
I'm not convinced for guitar. Some of the fastest and most famous guitarists had shockingly bad technique.
As long as you're not injuring yourself, practice and determination pretty much overcomes everything.
The universe didn't offer a manual on how to play guitar, so how are you determining that their technique was bad? Given what you say about them, maybe they actually had the perfect technique?
Given a certain set of quantifiable measures that is no doubt true, but then that only pushes the question to how are the measures determined to be objectively relevant? If the aforementioned fast/famous guitar players had started with a different technique there is a chance they wouldn't have become fast/famous. In that case, given the criteria of reaching notable speed/fame, it is possible their "bad" methods were actually best of all.
But also, even where everyone agrees there is a better way, that doesn't equate to an alternative being bad. So the original question still stands: How do we determine "shockingly bad" as opposed to "different"?
Piano is also a lot harder to have very bad technique than other instruments since it is mostly a discrete, one-to-one mapping. If you want a chord with some particular notes, a lot of the time there is only a single fingering that will do. If you push the key hard enough, you will get the correct note, and it won't be wrong if you push harder.
By contrast, in guitar, if you push too hard, your note will be off even though you fretted in the correct place. Or, for example, everybody in guitar teaches barre chords up near the nut, when that's extremely difficult and likely to injure a beginner who has neither the strength nor control to get that right, instead of teaching barre chords near the body on fewer strings. etc.
If you mess up, redo the part you messed up correctly 5 times in a row.
And, don't just practice the easy stuff. You have to challenge yourself to grow.
I think it may be important to note _when_ to redo this. I started off this way, but after working with a guitar teacher (a Berklee graduate), he recommended that I continue on with the song and return to the problematic parts afterwards. If you constantly stop at the problematic parts to replay them and get it right, you'll have no idea what other parts you'll have trouble with further into the song until much later. In addition to that, being able to move on and continue playing the song after making a mistake is an important skill itself. If you build that skill, it's usually only other musicians that will notice -- a regular audience won't.
What's your take on it?
Practice makes perfect is a thing, but that's not exactly rehearsal.
With practice you expect to improve, broaden, or maintain instrumental or musical ability for the long term. There should be no deadlines or need for actual listenability.
OTOH rehearsal is the run-up to a smooth listenable performance with a decidedly short-term objective by comparison. Unless you are rehearsing to absolute perfection, you do not halt for anything, the show must go on and that in itself requires you to practice covering up and compensating for your mistakes or shortcomings as you go along.
With practice you are actually trying to become a better player overall, but rehearsal is more about making the next performance as good as it can be and that's it.
If you're not actually as good as you would like in either regard, having a bit of commitment to simulating what you need most can give some direction itself to add to the mix.
Playing through the whole song from the beginning over and over again is not an efficient way to learn a new piece of music.
That transitions into a more rhythmic chord progression with a few embellishments, which also isn't terribly difficult at first, but then a few verses later that ramps up a good amount and becomes moderately difficult.
If you stumble on that transition / passage, then that is where you would stop and then practice until you do it correctly 5 times.
The next day, once you've rested and let your brain absorb the info, try it again and you'll find it much easier to get right on the first try.
Stopping and working slow is the only way you'll find and improve hard things. If you just blunder past them each time, you're only learning to blunder. Able to play the easy things but never improving the hard.
But if all you do is stop at every mistake, all you're learning is how to stop at every mistake. Live music doesn't stop, you need to know how to pick up and keep up, no matter what. (This was my mistake for decades.)
There's a lot more to learning a piece of music, but I think both kinds of passes are necessary. Well, unless you're good enough to fly through that piece prima vista with results that you're happy with. Then you get to hone the expression or interpretation or just cash in, I guess.
Everything takes twice as long to learn because I first have to unlearn the old habits.
And as my music professor once said: "If you sound good while practicing, then you're not practicing."
why am I posting here instead of practicing?
There's another, similar saying used in fighting-related disciplines: train how you fight.
Same idea. You're building muscle memory and technique, so make sure your training/practicing matches how you'd do it in a performance (or fight). It's one less thing to have to think about when you're under stress.
And at least for me, frequency beats duration. I make more progress when I play consistently for even 10 minutes every day than when I play for 90 minutes on Sunday afternoon.