this might be a silly question, but anyway: pretty much every article discussing burn-up and burn-down charts states that in the latter, scope changes are invisible because the chart only shows how much work is left to be done. But I do not understand - the scope creep should appear as a spike in the remaining work, shouldn't it? I understand it could mask progress (if e.g. a day was added and burnt at the same time), so in automatical reporting it might cause some issues. Otherwise, if my scope gets bigger, I would simply update the remaining work which would manifest as a spike in the burn-down chart.
The answer resides on the Y axis, which displays scope. On a burn up chart, when scope is added, your plan line will increase and will be visible on the Y axis. If scope is added on a burn down, the result would appear in the X axis, or extended duration. Extended duration most certainly is an indicator of scope creep but also indicates other drivers.
Changes in scope are less visible in a burn-down because a single line is used to indicate both progress and new backlog items. A burn-up separates the two so that the relationship is clearer. Burn-up also shows the actual velocity whereas burn-down does not because progress appears to slow (or reverse) whenever new items are added or future estimates are changed.
Be careful about using the term "scope creep". Formally speaking, new scope is only "creep" if a change is uncontrolled/unauthorised. A burn chart shows change in the backlog or in estimates but that's usually a positive thing and is not at all the same as creep.
The key difference is that the burn-down chart has a fixed 'expected' slope, whereas the burn-up chart can change the 'expected' target line when the scope changes.
So, you are quite right that the 'actual' slope will change in both cases, but the 'expected' will only change for the burn-up chart.
Perhaps a more accurate way of describing this would be:
Scope changes are reflected in the expected target line of a burn-up chart, but they are not reflected in the expected slope of a burn-down chart