Let's say you have a Stellium in the second house. If Saturn in transit is conjunct to the Stellium than it would mean (most likely) that Saturn would be in the same house and therefore cause problems also for that house. But let's say Saturn is in opposition with the Stellium, then Saturn would be in your eighth house (most likely) and cause potential problems there.
When Saturn in transit (or any other slow planet) aspects a natal planet, this also causes problems/changes to the houses the natal planet rules in the Chart. Let's say one of the planets of your Stellium is Mercury, then the houses with the cusp on Gemini and Virgo in the chart will also be affected. If the Sun is another, then it would be the house with the cusp on Leo and so on. Hope this was clear.