First, before griping about problems, I would like to say that Eternity is the one DOS engine I have found that is essentially perfect. Legacy crashes on me, Vavoom is very slow, unstable, and sometimes just not DOOMesque enough, BOOM is awesome but has no mlook or hi-res, SMMU crashes when I save, I can't find MBF or DosDoom [and neither are developed any more, are they?], EDGE doesn't run on my PC because it's too slow, and ZDoom has no music. Those engines all had one large problem that either physically prevented me from playing, or something that just bugged me too much to enable me to play - i.e. bad performance or a lack of a feature. Eternity, in addition to having what I needed, also had some other neat features - such as the start map and allies.
But, of course, when I initially see something as perfect, I very quickly find flaws in it.
Here are some of my gripes about Eternity - none are major enough to keep me from using it, but none are minor enough that I didn't notice them.
(a) You can't select the episode whenever you start an IWAD based on the Ultimate DOOM instead of DOOM II. I suppose very few level sets exist now that use the episodes instead of MAP01-MAP32, but they do exist.
(b) On some level sets that change the skull cursor on the main menu (so you can do New Game, Save, Load, Features, Quit, etc.) to something else, the cursor doesn't appear when you hit New Game and it asks you for the difficulty. For example, I notice this in Alien Vendetta and Icarus.
(c) The crosshair seems to have some problems, as follows - when you aim upwards, the weapon shoots much farther up than the crosshair indicates, and when you aim downwards, it shoots much farther down than the crosshair indicates. I was messing with Twilight Warrior (which said it needed Legacy but worked fine with BOOM, ZDoom, and Eternity) and I saw this with the sniper rifle and grenade launcher. I tried with normal DOOM weapons and they showed this problem too. Under ZDoom, this problem does not occur - the weapon hits right at the crosshair as accurately as it can.
I can make up some screenshots and demonstrate what I mean if you want me to.
(d) I would kinda like to see the ability to jump in the game, although just for a few levels that are meant to be used with ZDoom/Legacy where you have that ability but work fine with Eternity, until you get to the point where jumping is needed at least.
[wow, I only noticed one of those problems nearly immediately, and that was (b)]