I think there are a few threads here about VFC SCAR-L and the few issues appear to be mostly about the very thin wiring; the excessive amount of wiring; and the limited battery space due in part to the excessive wiring. For that, you can remove the stock wiring and get proper length 18 gauge wiring so that the wiring isn't as brittle.
Another likely issue is the infamous V2 gearbox shell cracking after many rounds fired or cold weather usage. For that, you can prolong your gearbox shell's life by installing a sorbethane pad from http://www.airsoftstore.ca/index.php...pjflh5io2v2da1
That being all said, there's no need to upgrade, maybe except for the easy PDI hop up swap. I run my SCAR-L stock with no upgrades and it's great, hasn't let me down.