John Arendt aka Beedo asked about flat-rate XP and sandboxes. I have opinions on this, because of the game I run and the game I play and a game I have played.
Here are three ways sandboxes can handle XP - by rewarding in-game action, purely flat-rate, and by story advancement/goal accomplishment.
Dungeon Fantasy: Felltower is a limited sandbox, and runs with goal-oriented experience. The system has gone through three iterations, but essentially, the reward are for loot (primarily), exploration (secondarily), and miscellaneous goals (avoiding death, finding special reward areas, etc.)
Since rewards are based on actions, especially loot, the game is very loot-driven. PCs make decisions that might be unwise from a long-term sandbox for per-session XP reward reasons. For example, the PCs have demolished several potential allies (even former allies) to get loot from them to reach their loot threshold and gain XP. They've allied with clear enemies to their own long-term detriment because it promised loot now.
Gamma Terra is a limited sandbox, and runs with flat experience. We get 5 xp per session, every session, no matter how much or how little we do.
We also get a random bonus every five sessions played, ranging in value from 2 points to as much as 20. So a reader might assume that skews things. But it's random, it's based on attendance not actions, and it doesn't skew anything except attendance.
Essentially, though, getting flat XP per session means we do what the situation demands. We didn't get XP for negotiating with the badders. We didn't XP for fighting them, earlier. We could have talked to the Iron Men instead of defeating them. We could have turned on the Triumvirate instead of allying with them.
The fact that XP is not tied to in-game actions means we don't take any in-game actions in order to earn XP. XP is totally divorced from our actions so we take actions we feel fit our goals in the sandbox.
The Known Worlds/Blood Dogs campaign featured story-based XP. As the PCs made progress by accomplishing in-game goals - clearing areas, finishing off story arcs, etc.* As the PCs finished what was essentially an area-based series of activities, they earned XP. Loot, slaying monsters, negotiation with potential enemies or allies, etc. weren't individually rewarded. Getting the group - the Blood Dogs - closer to their goal of defeating the evil wizard they'd unleashed and made central to the game was rewarded as they dealt with a block of linked issues. Plus I gave a small, minimal reward to players that attended each session to bulk out the rewards. But the largest percentage came from the story-arc rewards.'
Because of this, the PCs didn't worry about money except as they wanted or needed it. They didn't fight monsters except as they were obstacles. They didn't make friends or enemies except as they felt they needed to (and in the latter case, by their actions.)
Overall I think a sandbox works just fine with flat rewards. If you reward in-game actions, you get in-game actions centered on earning those rewards. A sandbox in an old-style D&D game will involve PCs aiming for loot. One in a later D&D game with most XP coming from monsters means they'll focus on slaying. A sandbox in a GURPS game with purely flat XP rewards means the PCs will just do what they feel like doing (or feel they need to do.) But it's totally doable do any of those three. My recently played and run games have been sandboxes, and the method of XP reward has affected how the players interact with the sandbox.
They all work, but they all give a different feel to play.
* I've said this before, but it bears repeating - "story-based" doesn't mean railroad, and doesn't mean a GM-centered story. The PCs essentially drove the plot with their actions, but it was the plot that determined their rewards.