I understand that the Plugin API is not everyones cup of tea. In fact at least some programming knowledge is required to use it, and a programming language like Java is usually always considered as "more complicated" than a scripting language like Lua. However, it's not intended that basically "everyone" has to use it to tweak his server or game, instead it's meant to encourage programmers/modders to write plugins for the game and make them available to the public. From this point of view it's actually much easier for modders to use the Plugin API instead of the old Lua API, especially since the new API has a proper documentation (which enables - when used in an IDE - handy features like code completion etc). In the long run, I'm confident that there will be more people using the Plugin API. I'm sure we will see some very interesting plugins in the future, especially when the API becomes more powerful (e.g. once it allows custom items, recipes etc).
Another huge advantage of using Java instead of Lua is the much better performance. Lua does not support multi-threading, but the RW server highly depends on that. And since there is no native Lua support for Java, using Lua creates a lot of overhead, since everything needs to be converted into "Lua values" (while the "Java values" and objects can be passed directly to the Plugin API without having to convert or wrap them).
Our old AreaProtection script exposed some of these performance limitations: if a server has a lot of areas, it struggles handling more than 10-20 players...
And since the API uses Java, it's easier for us to expand it. It already offers some nice features (custom images, models and sounds) which are not available in the old Lua API.
In general, having an API (no matter if it uses Java or Lua) is always favorable in terms of modding. When a new update for the game comes out, old plugins usually stay compatible. There are only very few cases which require plugin creators to update their plugins (e.g. if there were some extensive changes made to the core game). If there was no API in place, nearly every update would break existing mods. In addition, installing a plugin is easier than installing mods (unless there are tools like mod managers available).
And, above all: when playing multiplayer, players don't have to install any plugins on their machine. They simply can join the server without having to download or install anything manually, the game takes care of everything automatically. So server admins can simply add or remove plugins (or make any changes to them) without forcing the players to download anything or change any settings on their end.
Basically we try to offer the most important features "out of the box", without having the user/admin to install any additional plugins or scripts. That's why the "server.properties" grants access to several settings and there are various important features availble, like permissions or custom journals. The only "critical" feature that's missing is some sort of world protection (although solely the permissions provide some rudimentary world protection). Originally we decided to rely on the API for that feature, that's why we released the AreaProtection script in the past. Having a plugin or a script allows full customization, while there is little to no room for that if it's a core feature in the game. Unfortunately there is still no AreaProtection plugin available, and that's a pity. @Miwarre created a very promising AP plugin which was nearly ready, but he hasn't been online for over a year (I really hope he's well). Unfortunately there we had no time to create our own AP plugin yet...
Which particular plugins do you use exactly? Afaik most plugins available in the forum work pretty well, so maybe something went wrong with install it? But of course there are no guarantees that a certain plugin works properly, people create plugins on a voluntary basis, and since our community is still relatively small, there are not many plugins creators out there.
If a plugin outputs an error in a foreign language, it's probably because the plugin creator set up this error message. And of course there is always a change that a plugin contains errors, or that it triggers a bug in the game. The API is still far from being complete, but we're working on it 