You don’t want to be flipping houses in a market which is having heavy falls in price because you don’t want to be in a position where you secure a property and you cannot achieve your end price once renovated because the market is weaker than when you purchased.
In saying that, all suburbs in a city act independently, so just because overall a city may be experiencing price falls, there are usually some suburbs which are still performing well and having moderate growth. Therefore by using data you can be more selective with which suburbs you are choosing to flip in.
On the flip side, in slower markets there is less demand, buyers are wary & the days on-market extends. If sellers need to sell, it can mean opportunity for buyers and bargains can be had, so, as long as you are buying really well, potentially you can still do really well in a slow market.
We shouldn’t get a neutral market & a falling market mixed up. Flipping in a neutral market is totally fine! The market that you live in is not always going to be increasing in price. In a neutral market, houses that need renovation will not fly off the shelf like they do in a hot market & they tend to sit on the market, again this can be great buying.