Multipools have a detrimental impact on smaller coins when they shift over to them to beginning mining.  They can easily fork chains of smaller coins when they induce their massive hashrate.  Secondly, they can kill the market for these same coins by dumping millions of coins on the market in an attempt to "pump and dump" so that they can move onto the next coin.  This causes a massive deflation in the coins price.

Coin devs are recognizing this and have begun implementing KGW and DigiShield like variants.  This prohibits the multipools from staying on the coin for very long as they diff rises sharply once they arrive.