Advanced rocketry, it is possible to build a rocket with one engine, but when you launch it does not go to space. When scanning rocket you have to check that acceleration is greater than 0.00.
https://github.com/zmaster587/AdvancedRocketry/issues/1233
IHL multiblock machines. These are hard, there is a spot for motor, it matters how you place the parts etc.. I recommend mentioning in the quest book to check the FTBU manual for instructions how to build these and perhaps are even more info there. I noticed the FTBU manual only later in the game after I had figured all of those out with a lot of googling and trial and error.
Detonation spraying machine. There are two different flint&steel in the mod. And only one of them works with this. Please mention this in the quest book if you can't fix it so that there is only one flint and steel.
Some minerals are hard to find and require information about rock types. I had to find and read the configuration files to figure out where the mineral is located.
Finding minerals is something really frustrating and really hard. Even when you advance in technology you don't get any better with locating new minerals. I wish there was some tool you could make somewhere in the mid-game which would help locating minerals. Possibly another tool in late game.
I'm not sure if this is issue in normal gameplay, but when I tested steam turbine controller in creative mode, I could not get it to work. After hours of trying I finally noticed that there are 3 different steams in the game and not all of them work with the machine. Renaming items that you are not supposed to use might help?
There are multiple situations where one needs to seek help from the internet. e.g. space flight entry in the quest book does mention several items you need, but it doesn't mention the amount nor does it mention all of the times, e.g. the chip you put inside guidance computer. This is somewhat OK, as long as help is easily found from the internet, but it would not hurt to improve this by being more specific with the amounts and items that are actually needed. Same issue was with oil refinery equipment.
Electricity conversion. Considering that the modpack contains multiple mods that use different electricities, it would be fair to give a better way to convert from one unit to another.
Some things that are hard to figure out: