Your iPad controls charging of its battery - not the USB power source. That said, the USB power source must be able to provide sufficient power to charge an iPad...
Most models of iPad require a Power Adapter or other USB power source rated at 12W (i.e., 2.4A @5V) or greater to reliably charge. 5W Power Adapters (1.0A @5V) intended for mobile phones are inadequate to charge an iPad; if the iPad charges at all, charging will be very slow. Recent models of iPad are supplied with a 20W USB-C Power Adapter.
Some third-party USB-C power adapters (and portable PowerBanks) may not support USB PD (Power Delivery). Power Delivery mutually negotiates the charging voltage; the source, load and cable are all elements involved in the negotiation protocol. Unless the Power Adapter explicitly supports USB PD, it will almost certainly fail to charge the iPad; proprietary charging standards, such as QC (Quick Charge), are not supported.
Many computer USB ports also cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, computer USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).
As for being "safe", be aware that USB power outlets found in public locations (such as Airports, transport hubs and hotels) can be modified to represent a security risk to your iPad and other computing devices. As such, it is generally recommended that you avoid this type of power outlet. Instead, best practice advice is that you should only charge your computing devices with your own Power Adapter connected to an AC Mains outlet - and/or a portable PowerBank battery. Always use your own USB cable to charge.
