Is there a practical reason to go with a custom-made solution when you can choose among tens from-the-shelf systems, where you can tailor "trading system" by several clicks? We believe that there are three key points to consider
All trading platforms(standalone or cloud-based) is simply yet another layer(usually - layers) between actual strategy logic and trading destination. In real HFT there is no place for anything that could slow down decision-making or execution. We are optimizing every step when building strategies from scratch(well - almost from scratch), getting the most from the hardware and excluding(or moving to the another place) all operations, which could add extra latency.
Your strategy is working on your hardware and controled only by you. So you are the sole beneficiary of its performance.
Unfortunately, no one can't guaranty same behavior in the cloud-based environment(despite its declaration). Your trading operations can be copied through the MAM modules and even source code, uploaded for execution and can be reused.