Having experience with the loot system and leveling scaling, and having unlocked all the safe houses at an earlier-than-expected level, the only thing I could argue as a benefit of exploring high-level areas as a low-level agent would be the bragging rights.
It depends on the exact level you are going after, here. If you want the challenge of facing up against enemies 2-3 levels higher than you, go for it. It will likely take longer to level, if you account for the extra time it will take to kill each enemy and the unavoidable increase in having to re-spawn, but the bragging rights are there. It does not take long for the higher level to mean instant death.
Collectables and Vanity Gear
If you collect every collectable in a category, you are awarded with a unique vanity item. For instance, if you collect every phone recording, you will be given the Shoreline jacket. It might be cool to be the only level 10 running around with one of these on, but considering the extra exploration that goes into collectable hunting, this could easily be a painful task.
You earn more experience in high-level areas. This is both a result of the higher rewards for missions, and the higher level of enemies. In my experience, the difficulty increase draws out the time it takes to level, considerably. It is actually much quicker to progress through the missions suited to your level, as you are able to achieve the experience reward much quicker.
You might still find lootable chests, however, you run in to a few more issues:
- From what I have seen, gear drops are leveled to the character, so your still going to find the same kind of gear as in the lower areas. If I am wrong, your most likely to find average gear for the intended level group. So you either find average gear, or average gear that you won't be able to use while it is an upgrade.
- Enemies will destroy you. At level 25, a level 30 enemy could kill me in one shot. Enemies were also more drawn to me, and seemed to be able to hit me way outside the range at which they can hit me, now I am at a matched level. Even if you are able to down an enemy, and lucky enough that the enemy drops some gear, you still have the same problem of the gear requiring a much higher level for use.
- Material and Consumable chests will provide the same items, regardless. That includes chests that give you crafting material. For the purpose of farming these items, a low level area is more desirable, simply for the lower danger you will encounter.
- Contaminated Zones are more likely to be outside of your reach, making it less likely that you will be able to venture in to these areas for some easy gear chests.
You can unlock quick travel, yes. However, you have no real reason to quick travel to the location, until you are within the level requirements, or higher. Missions will be difficult, enemies will be difficult. Yes, you will earn a lot more experience then from the missions suited to your level; but it will also take a lot longer, and your leveling over time will not be as good as if you started on the challenges suited to your characters level.