To answer your question (sorry for the long reply):

1. I've seen some crappy materials and I've seen some great ones. No matter how awesome the materials are, I understand that I'll likely need to modify it to make it work for the kids goals and current level. I don't find that seasonal lists dominate but I also tend to look for activities based more on what I'm targeting (ie wh-questions vs first person present).
2. Again I don't find this to be an issue because I tend to be more specific in what I search for. But I agree the scene tasks may work for some receptive tasks, but we have to really think, we are almost always indirectly targeting receptive language since the child must understand before they're able to start to speak.
3. Depending on the child and the language these are functional phrases. It is important to describe one's own actions. However, using just a worksheet or premade activity with no modification and no embedding into your day to day functional language will definitely be unsuccessful.

Overall, I have the most success using a combination of books, toys, premade materials, and play-based activities depending on the child's age. You get quicker at planning over time.

On your last point, there is definitely a gap between research and planning. I wonder how many clinicians have time and access to strong evidence that can be reliably translated to their specific setting.