About the telltales in sails, I will also agree that good sailors can "feel" the sail (flow) really really well.
But even if they can, telltales will still be able to give some crucial information that can not be felt the same way.
Because, every sail needs to have the correct amount of downhaul (kicking strap) in order to get the correct twist (AOA changing from the boom up to the top of the mast).
The wind has a very different angle at the lower end (around the boom), compared to around the middle and upper part
This angle changes with changing wind strength, and point of sail (also sheet tension), how gusty it is, the stiffness of your mast, and sometimes even water surface conditions.
Meaning, you can change your efficiency and speed and upwind angle by a margin by having the sail twist "perfect" - not too much nor too little, in a very given scenario that changes over the course, thus not fixed.
Having telltales at the lower and middle and upper part of the sail is really crucial here
In a kite I find it very interesting to see when the flow separates and how often.
Also, how far out on the lifting surface will you destroy the flow when doing even small bar corrections ?
We all know that the tips of a kite is sometimes fully stalled and the other side fully luffed, when doing a tight kiteloop.
They are not delivering lift though - but as soon as you go towards the center of the kite, you will also have "twist" and maybe a lot of inefficiency even with small control corrections when flying in gentle sinus'es ? Or not - that is where telltales can help visualize.
Some kites tends to have too much or too little AOA on the chord at the center compared to the middles (right and left middle lifting areas) so a lot is "lost" this way.
Here, telltales can reveal when very off
"Feel" is NOT possible here - as no matter how good you are, you can only feel the best AOA for a given scenario with a given kite - but as in the sailtwist scenario, you can NOT feel if the AOA is wrong at the center of the kite compared to the middle sections.
Personally I am not much into slots and vents, as it is IMO a considerable performance sacrifice one makes in order to make things "easier" to handle, and not a thing you do to gain anything except that.
Flaps is different, as they can actually make things perform better at high AOA's as in turns, with minimal loss really, and perform better at low AOA's too.
Regarding the "boundary layer" turbulent airflow, yes, correct that it has a significant thickness (compared to laminar flow only a mm or so in thickness).
But I think that if the telltales are not too short, they will still be able to detect when separation occurs - and THIS is the point from where lift is lost and drag increases by a huge amount.
It seems, according to what JS observed till now, that flow is disturbed into separation a lot more than we (I) think it would be
But a kite is designed to work "adequate" with a lot of control, and really a simple construction, so these areas with semi separation, not a stall nor fluid flow either, are maybe widespread and common ?