The last stable 412. Why do they call it stable? Is it that it was the last version that passed 100% unit testing coverage?
Everything after is minefield of bugs and frogs falling from sky exploding in your face while cvet laughs maniacally behind your back while trying to cut your hair with a wooden plank.
On serious note~
Bugs are plenty, and can remind SDK pre-release times for some. Some are funny, some horrible, and some are not possible to workaround without hacking; best example - somewhere around r41x/r42x client stops loading dlls. It's a matter of few lines to fix that, but not every scripter is capable of writing dll injector just to change one specific bool (not to mention finding it without source).
Refactors every few commits; smaller, bigger, pointless, you name it. While they are not a big deal when you just started making Best FOnline Ever, they can be overwhelming when you have years of content already ingame.
And mind you, that's FOnline part only. There's AngelScript too, which is completly different story. While Andy usually tries to keep transition from new to old smooth, FOnline is compiled to always use newest stuff. Don't remember details now, but there was one specific change which affected casting int to float, or float to int, or both... Such things are usually around combat related/fancy math code (but it can be anywhere really) and you have to deal with them on par with previous two points. Fun, fun, funline.
But wait, there's more!
After you do all that and still have some sanity left, players gonna get...
Dropped DX support, which might or might not affect your playerbase. If majority still uses DX client for w/e reason, you have a problem
Overall drop in perfomance. And that comes from Kilgore who always been making fun of my wooden computah machines. Beside him, there was multiple reports from players during short test of newer (back then) client on Reloaded.
This one is most likely outdated, but there was few new shiny crash points which has never been taken care of. After two month of waiting for one thing to fix, reported many times, you simply decide to switch back to revision which doesn't provide all that shiny stuff, but simply works without all that problems.
And that, my
little great old one, is how you fall in love in revisions <= 412.
AoP is a good example that staying one revision can open a whole new world of possibilities. When you know client inside out,
great stuff can happen. Or cirn0. Either way, you can then focus of making/improving content and/or forcing client/server to obey your will, without need to worry about incoming changes.
There are cons of that decision, ofc, but i'm here for r412 propaganda, not objective opinion
Overall, i don't see any long-running servers to go beyond 412. What cvet pushes into FOnline is awesome for a new project, but not for one which has many years of history. Unless you have solid team of update-focused devs, and not just few people doing FOnline for fun, as a side-project. And nearly all FOnline servers are exactly that.