Overnight trading is substantially riskier as trading a very large pooled number of contracts (large trading size) makes it almost impracticable to exit positions during the very low liquidity overnight future markets. If you were to exit a large position overnight, you would suffer extensive slippage on markets such as Nasdaq Futures and Dow Jones Futures.
Quant Savvy algo systems are day trading only, as holding overnight negates the initial positive expectancy on the entry and exit. The longer the trade, the more trivial the entry becomes and the ability to have a large trade sample is greatly diminished when creating the initial system.
System providers who display overnight systems have often never traded a large position size and often created only backtests based on highly liquid scenarios which are misleading and distorted in reality.