The scalability like you mentioned is also an important concern. Until today no DEX has ever tested to handle massive transactions and its capability is still questionable. But to do that test we need to solve the first problem which is the low trading volume and the lack of users. it's like DEX got stuck because it can't get anywhere with that kind of low trading volume and the development can't really progress. We've seen some problem that occurs in bitcoin's blockchain after it got overcrowded and the transaction requires a high fee and still take so much time to get confirmed. If we can make a conclusion by some simple logic, DEX could actually face the same scalability problem and need fixes as to how bitcoin implement lightning network.