Render Engines - A confusing dilemma

The situation for me is as follows.

So there are two render engines with blender (by default) as far as my understanding goes.
“Blender Render” and Cycles.

Some thoughts that are on my mind:

-Cycles has nice features like GPU support which makes it way faster than Blender Render.
-Blender Render has been discontinued(?)
-Yet, over and over again I stumble upon things where Cycles does not support something yet (certain particle types)
-It is hard to find guides on how to do stuff for a specific engine, especially with something named as generic as “Blender Render”
-So what do the devs want me to use? It seems they want me to use Cycles, even though both engines come from different perspectives (realism vs. not iirc?), and even though the feature set does not seem to be there yet(?)
-Is there a list of things Cycles can NOT do yet compared to Blender Render? I don’t want to continuously stumble upon issues I cannot solve when using Cycles.

Thanks for your time.

Cycles and Blender Render (aka Blender Internal), have different ways to render images.

BI method is not versatile enough to mimic the way light interacts with objects. But it can produce good results.

BI is not discontinued, but much of what the engine can do it’s already there.

Cycles on the other hand can’t deal very well with non realistic materials. And some effects are quite difficult to perform in Cycles without breaking the whole logic behind it. (bildboard particles, for example, make more sense in BI)

My advise is to learn a bit how to work with both. For realistic lighting and materials, use Cycles. And for the rest, or if you need some special elements for later composition, use BI.

So I should usually stick to Bl and accept that rendering will be much slower because there is no gpu acceleration?

Bear in mind that cycles has much more active development now. We have GPU support for volumetrics for example, and smoke, which were a big let down before. Cycles is still very much a work in progress, but progressing nevertheless.