For the record it is a flash cart and not an emulator. The game you are running is running on the hardware it was designed for and thus does not count as emulation. Some systems can have certain problems if the original carts had extra hardware (NES and SNES being good examples) but the GBA is pretty simple. For some it is a minor distinction, both are means by which to play your chosen code aimed at a given device after all, but I would hold it is one worth making. Unless you literally mean you want to play gameboy games (as in GB/GBC), in which case all GBA flash carts you will be able to find will use emulation to play those.
Ask if you can play the vast vast majority of GBA games just fine on an EZ4 and the answer is yes, yes you can and thousands of people have done for many years now. There are games that cause trouble, most of those have patches to work around that. More
http://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-18#post-4756995 .
A lot of patching? You select the ROM, press go and it sends it to the microSD. We could debate what counts as a lot of patching but you would be hard pressed to count the average GBA patch as that (DS games often had to be individually patched, GBA ones maybe had to have the patches remade when a new save type appeared but were otherwise fine as the concepts were that generic for most things.
Buggy? I can get them to behave oddly, I can get most electronics and programs to pitch a fit if I want to. Can you simply turn on, select the games option, select from a large list of things and play for 90% of stuff without any contrivances and having to remember things? Sure, and for the rest (the greater than 128Mbit stuff from the thread linked) it is a minor thing (you make sure you have space on the NOR and press select to get it to burn there, at which point it stays until you delete it).
Is the everdrive easier still? Yep, however the EZ4 is pretty braindead simple to deal with and operate.
