Gauntlets, security, accessibility and coordination. Plus falls and loot.
Assumptions: A teleport scroll will only take a single adventurer and the items that adventurer is wearing / carrying. If this assumption is incorrect then there are lots of potential problems (how much of the ground they are standing on is teleported with them?) and a very simple solution to why teleporting is a bad idea (the hostile touching the adventurer will teleport with them and continue killing the adventurer post-teleport).
Gauntlets - Hands are vulnerable. When facing an enemy, an adventurer's hands will typically be the closest part of the adventurer's body to an enemy. If an adventurer's hand is damaged then their ability to fight, treat their wounds and conduct a myriad of other tasks are seriously impeded. Therefore, melee fighters will wear protective gloves or gauntlets to prevent themselves being disabled. Archers using traditional war bows need a protective glove on the bow hand to prevent damage from the fletching of the arrows being shot and a half-glove at least to protect the fingers of the drawing hand from the bowstring. Even spellcasters, assuming that there are such, will find themselves needing gloves in a number of environments and adverse weather.
Even light, modern gloves designed for flying and running reduce the wearer's dexterity for tasks such as tying and untying knots and bows, removing items from pouches etc. Heavy gloves and gauntlets make such tasks almost impossible.
The question states that:
To prevent one from accidentally triggering the effect the scroll is rolled and tied up with a ribbon.
How long does it take to undo the ribbon and tear a scroll? I ran a few tests using sheets of A4 paper, rolled up, tied with a ribbon and placed in an unfastened breast pocket of my jacket:
- 5 seconds when using both hands with no gloves under no pressure. The paper did not tear remotely cleanly in half - scrolls need to be perforated if a clean tear is required.
- 8 seconds when using both hands with snow gloves under no pressure.
- 17 seconds when using off-hand with snow glove and my teeth while wrestling with an energetic adolescent 25 kg German Shepherd X with the primary hand.
- 11 seconds when using off-hand with snow glove and my teeth while (badly) practicing parries with an ornamental short sword in the primary hand.
One interesting observation was that my form while parrying in the last test was even worse than normal due to splitting my attention between sword work and trying to unwrap and tear a scroll. Against an opponent who is so superior that I would need to teleport to safety, I would not survive the 11 seconds it took to access and tear the scroll.
Note also that I did not even attempt to simulate a two-handed weapon, dual wielding or weapon-and-shield fighting - one hand must be free to access and unwrap the scroll, which means that a two-handed weapon cannot be employed effectively while unwrapping a scroll or an off-hand weapon or shield must be dropped and presumably abandoned (see "Loot" below).
Security and accessibility - In the above tests, the "scroll" was not secure. An enemy could pick such an obvious pocket easily, it would not be a feasible place to have a pocket in most types of armour and any flame, slashing or piercing attacks would destroy or at least damage the scroll before it could be used. In order for the scroll to survive to be used it would need to be in a more secure pocket or pouch, which would mean that instead of 10+ seconds to trigger the scroll it would take much longer. 10 seconds is a long time in close combat when things are already going sufficiently wrong that it's bug-out time, longer will make it that much worse.
Coordination - solo adventurers are not particularly plausible as a sustainable operating model. Adventuring teams are needed in order to allow for role specialisation, all-around observation and any teamwork tactics (eg distractions, flanking). When an encounter has gone so badly wrong that the team needs to teleport out, how well will this be communicated in the fog and din of battle? Will all team members realise that it's "scroll time" if they cannot hear the order to teleport? As soon as some team members teleport out, the adventurer/s remaining will face the full brunt of the hostile encounter.
Falls - Adventurers may fall to their deaths. Things may fall on adventurers to cause their deaths. Assuming that a really alert adventurer who is either falling or has something falling on them can access and tear their scroll in 5 seconds (with no "decision" time required) then 125 metres is the magic distance. If they fall less than 125 metres or if something is falling on them from a height of less than 125 metres then there is no time to use a scroll. 125 metres is really high - 99.99% of drops of or onto adventurers will be much less than this but still easily fatal.
Loot - Adventurers live and die for loot. They prefer not to die, which means that if they are entering battle while carrying heavy loot then they will put it down rather than try to fight with it on. (Armour is bad enough, but at least it is distributed over the body. A backpack with more than a few kilos in it will unbalance an adventurer, slow them down and restrict their movement - for example, it is simply not possible to fire a rifle or a crossbow from the prone position with a large backpack on.) However, this may lead to a possibly fatal hesitation when a battle is going badly - should they fight on and possibly die or teleport out and lose everything that they are not carrying? Note that this includes not only their backpacks but any weapons/shields of their own that they need to drop in order to have at least one free hand to access and use their scroll.
Summary: The time and splitting of attention to use a scroll may make fighting on or attempting a conventional withdrawal a more survivable option in many circumstances. When an adventurer falls or has things dropped on them they will not have time to use a scroll. Finally, adventurers may be reluctant to abandon team mates and/or valuables.