After having used dozens of Arduino boards, 10 ethernet shields, and some other modules, and having had all sort of issues with them all, I'd like to share my thoughts about them if they may be useful to anyone here:
Arduino Mega: Currently the clones you will buy will probably come with an "unofficial" Serial-USB chip. Originals should bring an "16U2" IC, while newer clones have an CH340, which, at first works fine, although will require you to manually find and install a driver for it. Also I got one which failed to get the automatic reset while uploading, I guess there was some issue with th trace between DTR and hw Reset line, or the capacitor value, but I never investigated it. I just pushed the reset button while starting to upload.
Ethernet Shield v1: I've had to buy a bunch of these module. I got in real trouble when most of them stopped working when I was at the office. They wouldn't want to get an IP, sometimes they would...then they wouldn't again... At home, they all worked 100%. I messed with libraries, code, everything, until I found the issue. Many chinese clones come with a wrong value 4x resistor pack. The small rectangle right behind the RJ45 socket should read either "510" or "49R9". You may need a magnifier, or use your cell phone camera. If it reads "511", you're screwed. Either return it, or solder 2 100 Ohm resistors in the bottom side, between pins 1-2 and 3-6 of the RJ45 plug. Those are the terminator resistors for the ethernet line, used in pairs give 49,9 (marking 49R9) or 51 (marking 510) ohms, but the wrong one is 510 Ohms...(marking 511). Some routers, like mine at home, seem to work with such an incorrect value, while some others (office), won't work with such a high impedance
Also with the last eth shields v1 I've bought I had the bottom solder pads for the RJ45 lights (at the edge of the board) are too big and get shortcircuited to ground when you stack the shield on the Arduno and they touch the metal on top of the USB socket, causing a reset. Also these last ones had the bottom row pins hand-soldered, which doesn't necessarily fail, but looks terribly bad and has some issues while aligning them to the Arduino socket.
Tiny RTC Hw clock module: This thing didn't want to keep the date/hours when it was disconnected for some hours. Then I discovered that the board is designed to use a rechargeable battery, which is not the case in the ones I got (and probably not many have a rechargeable battery). So they have a diode to charge the battery off Vcc, and a voltage divider, as rechargeable batteries hav 3.6 or 4.2 volts (can't remember), to scale it down to 3V. So you need to remove D1, R4&R6, and place a jumper between R6's pads. This fixes all issues with it...
I have also had issues with the SD (in the ethe shield) not initializing correctly at times, I think I got it fixed now but I will now shortly...
If anyone needs help with any f these issues, I can help if needed, just ask for details
Hope this can save time to anyone in the same boat....
Thanks