I think it's more about the ball being further away from the inside defender than it would be in the other hand... a fend or a step can get them further from the inside defender. If it's coming from a centre or a second rower, then they're putting pressure on the outside defender to come in, and then all of a sudden you've committed two defenders and an outside man is open.
Even if that is the case from coaches I just think it's so rare to get a high quality attacking player with a left hand carry that it is a waste to put them on the wrong side of the field.
It's ALWAYS a tradeoff.
Most rugby players of both ilks need to become reasonably ambidextrous in both legs and hands to progress through to the professional level. That applies even more-so with kicking, which is harder than hand co-ordination, because it requires hand to leg co-ordination.
Playing on the wing requires a few standard skills:
1. fast, usually the fastest in the team
2. hard to tackle,
3. safe under the high ball
4. stable under lateral pressure
5. evasive
6. freakish skill at catching bad balls at high speed under pressure
7. freakish ball placement over the line
8. a variety of inside kicks
9. solid defence against fast moving opposition
That's before you get into reading the game and fitness.
It's not as important for a wing to have an excellent passing game, but you will need a basic passing game, especially to the inside, including basketball style variations and long distance lobs (particularly in union.)
So if you're the fastest in the team, and passing isn't your strong suit, you will inevitably be looking at one of the wings. Then you will have to decide which one.
The more dominant you are on one side of your body, the more you'll be conflicted.
If you are right handed/footed, you will gravitate to the right, because:
1. You will fumble fewer catches if you can stop the ball with your right arm.
2. It will be easier to kick inside from your right foot.
3. It will be easier to step inside from your right foot.
4. You will have more strength to push off attackers coming from your inside with your right foot.
However, if you are right handed, you will be disadvantaged on the right, because:
1. You will naturally tuck under your left, and fend with your right. Defence will mostly be coming from your left.
2. You will prefer to hold the ball in your right hand when diving for the line, a disadvantage in the right hand corner.
That's it. You have to train to be as ambidextrous as possible, and then trade off your fend against your catch and kick. A dominant right hander needs to practice tucking under the right, and fending with the left, and placing with the left. It's not easy, but that's why you train.
And then when you master both sides, they might move you to the centre.