Safe to use both AC and DC with 4 channel relay?
Certification

In most El NEC territories (US and most other countries using 120V) it would not be legal to control 120V with this device, because it fails on NEC 110.2 - equipment must be approved.  Approval is done at the local level, and since towns don't want to be in the testing lab business, they defer to a list of recognized testing labs maintained by USA's OSHA.  The list features UL, CSA, ETL and a few others. CE is not a testing lab.  FCC, RoHS and CCC are also not labs. 

If the equipment had been approved, the manufacturer would show the hard-earned UL, CSA etc. logo with its 5-8 digit file number, as proudly as a daddy shows photos of a child.  Note that UL listed applies to products and equipment fit for use in mains wiring, and UL recognized (RU) applies to electronic components, unfit to use (directly) in mains wiring.

Part of mains wiring rules relates to physical mounting of equipment. Another big issue with this one is it does not provide any proper method to mount it and compartmentalize the electrical connections.  You could put the whole thing inside a large steel junction box, but then, the WiFi signal wouldn't get out.  

There is a veritable "junkstream" of cheaply made stuff coming out of a certain very large non-currency-manipulating country in the far east. It's marketed at AliExpress, Banggood, eBay, and Amazon Marketplace.  Where mains power is concerned, don't get anything from that junkstream. Anything! 

Separation

You're not allowed to bring mains power and low-voltage power into the same wiring junction boxes.  So even if this item were UL Listed, you could not switch mains and LV wiring with the same unit.  

But consider this.  There are UL-listed (mains legal) relays such as a RiB or Aube which have 5V, 12V or 24V coils, but whose contacts switch mains wiring.  They provide the required "Chinese Wall" separation between LV and mains wiring.  So you could have a Sonoff installed to be entirely low-voltage.  Some ports would switch your intended low-voltage loads, while one or more switch a pilot signal to a RiB or Aube relay.  And that switches the mains loads. 

The rules for low-voltage wiring and equipment are much more liberal, so doing smart controls in all-low-voltage right up to an Aube relay, is a great way to handle it.  An Aube can be a nice choice because it makes its own low-voltage from an onboard transformer, you simply bring 2 wires from it, and either connect the wires or don't.