The real problem is the inconsistency - effectively the game allows to use hinting with "adaptive mode" disabled and refuse hinting when enabled, but that's not that simple. I've described the bug in the ATS section (Shift Up/Down Hint broken with Allison Transmissions 
), but let me explain that more thoroughly:
1. Let's start with ETS 2/ATS, Allison equipped truck, "Adaptive mode" disabled, throttle operated by:
- "mouse steering including pedals"
-> shift up and down hint work as desired - the game accepts input, switches the gear, changes displayed gear number, changes engine rpm and then keeps the gear if engine rpm is appropriate, or shifts back if not.
Stop the truck.
2. Now let's enable one of "adaptive modes" and accelerate gently with mouse / pedal (no keyboard, no full throttle!) or with cruise control switches -> the hints still work! - the game accepts input, switches the gear, changes displayed gear number, changes engine rpm and then keeps the gear if engine rpm is appropriate, or shifts back if not.
3. Go full throttle for a moment with mouse, pedal or keyboard so the kick-down takes place and release throttle / activate cruise control -> now the hint controls are locked (disabled) forever*. The game does not accept the shift up/down hint input opposite to 2., there's no even brief displayed number / engine rpm change.
4. Now disable "adaptive mode" in options, back to the initial conditions. Try to use hints - they still don't work! They are locked and 4=/=1 as it should.
*5. Reload / restart the game or rebuy the transmission. Hints work again in non-adaptive mode with Allison transmission, they even work in adaptive mode until the next kick-down occurs.
Is the game behavior consistent? Has it been designed like that? I don't think so. There's some bug in the code which occurs at the time of kick-down activation which changes game behavior and locks controls. If I can recall correctly there was no such problem with hinting after ATS release, it was introduced later in one of the updates (6-8 months ago?), probably with some torque converter tweaking. I stopped using adaptive mode after that or switched to other transmissions in both games.
It's a bug, not a feature
So dear SCS, please, fix the bug and enable the hinting for all modes or disable it completely, so the game works always the same way.
Should you allow players to use hinting?
Some of you say that hinting with Allison is problematic issue, those transmissions cannot be hinted at all, the mythical torque converter works differently than other gearboxes, that it chooses optimal range etc. and in real life that's probably true, however...
The ratios between gears 4-5-6 of 4500 are 1.31 and 1.13
, between 4-5-6-7 of 4700 are 1.43, 1.35 and 1.16
, all ratios between gears in TC-10 are 1.16-1.36 (quite close to 1.26-1.28 of "auto" 12-gear gearbox in ETS2 which allows hinting). With predefined shifting points, while accelerating, the game tends to choose the lower possible gear and still keeps it while maintaining constant speed, so for example while cruising the engine operates at 1400 rpm (4700/6th) rather than 1200 rpm at higher possible gear (4700/7th) which often means higher fuel consumption. This type of situation takes place very often, not very rare as you might suggest. That's why I consider disabling the ability to hint the transmission a mistake.
One of the main highlights of ETS2 1.27 and ATS 1.6 updates is adaptive mode enhancement, as well as inclusion of more Allison models in ATS.
The Allison transmissions are advertised to reduce fuel consumption, have dynamic shift sensing and advanced shifting control. As long as the games don't really simulate these features it would be better to give players the ability to make up for the shortcomings with hinting, without need to switch to sequential/manual mode.
EDIT: fixed some values.