Unless otherwise specified, expect any generic USB charger or outlet to be a dumb 500mA USB 1.0, or even 100mA USB. Pretty outdated. Slow or no charging of larger/newer devices Probably not a good choice for hard-wiring.
You want a charger that specifies USB 2.0 (preferably 3.0 compliant), a high total charge rate (ideally 4.2mA), the capable of providing 2.1A per port, the capability of providing the high charge rate on multiple ports, and Apple iPad compliant if you really want a fully capable device. Also, higher charge rates are 'negotiated' over the USB, so a higher end outlet will have a microprocessor and software in it. Even devices that specify "2A charging" often do not have the intelligence or correct software to negotiate the high charge rate with many devices like an iPad or newer iPhone, and default back to a slow charge rate. Many (most?) also do not support the higher charge rate for a second device, which kind of defeats the purpose of having a multi-port charger.
A really cheap USB outlet will essentially have the capability of a 8-10 year old USB phone charger. Not what I would want to hard-wire. Even if it meets your current needs, it will almost certainly need replacing in the future.
Here's a link to a high quality
4-port and
2-port/2-outlet Leviton charger... They are both 125V, but it will give you an idea what to look for in an 250V outlet.
I personally like the idea of having an all-usb outlet rather than a AC/USB outlet. All the places I would potentially install one have an abundance of AC outlets, and the ones I would typically replace are pretty much only used to plug in chargers anyway, so losing one AC outlet is no big deal.
Example:
Leviton 4-port (125V) USB outlet
4.2 A total, 2.1A per port, microprocessor negotiation, Apple iPhone/iPad compatible.
Even this device can't 'fast charge' more than two devices before it hits its current max...