I was told that the more efficient way to move actors along splines is with Sequencer and not via BP code. Google-sensei is not cooperating (or there are indeed no tutorials / docs about it).
How would I move an actor along spline using Sequencer ? (on timer or on trigger; with variable speed if needed be; etc.)
In what cases using BP is preferable ?
Thanks
Anyone? 
looking into similar I need to move an animated mesh along a spline using blendspaces here but i think we are at the bit where google starts to wear thin
You should be able to use the spline actor with a path track. Add the object you want to place along the spline actor to sequencer. Click on the +Track for that object and choose the spline actor from the Path submenu. Once you do that, it should automatically create a float track for the Timing Curve. Set keys to indicate 0 - start of the spline, 1 - end of the spline.
Took a while to find what you were describing (never would have found it without this topic), but I have an issue with it:
In the sequencer, scrubbing along the path track seems to place the object with Y-forward, but if you get transform at a time using a blueprint, it places the object (correctly) with X-forward.
Anyone have any idea how to fix this?
In general, I’d like to know how to add relative rotation to an object in a sequencer on a path track… if I add a transform track with rotation, it just locks the rotation in place.
I am having the same issue with the actor being locked in a direction once a transform is added. I have been having a hard time finding a way to fix this. I have posted a question here
I ended up doing what you do in 3D software in general, which is to add a dummy object as a pivot point and parent it to what you want to move along the track. Then whenever you want to add a relative rotation, you just do it to the child, and the spline will still affect the rotation on the parent.
This will also serve as a work-around for the Y-forward vs X-forward bug.
I was having the same issue today. In my case I had a blueprint actor that I wanted to animate along a spline and have it into the sequencer. I managed to fix the wrong rotation by adding an arrow component (or a Child actor could be used as well) as my root for my blueprint (the one that will get the animation path) and make it a parent for my static mesh. Then rotate the static mesh 90 degrees. Now the static mesh has the rotation needed to follow the path fine 
Can someone add a screenshot here of the sequencer showing the spline actor added as a path here? That would be helpful
I dont know if this helps past posts or future searchers but anyway. I too had struggles with spawning spline and actor then make actor use spline points as travel paths.
My issues were I was trying to call my combo within a BP then on to the level.
YOU MUST in this case Make clear Variable References to the Track(Spline) in the Cart(Actor) -the track has nothing inside this BP besides a Spline component.
Your Moving Actor uses the refence create of the spline to target this specific track right after SpawnActor. You must link your SetRef of Spline From the return value of the spawned spine.
I created a timeline within the Cart to set playrate/speed etc.
if you need help follow Cart and Track videos on uTube.
does anyone know how do i make the spline path not trigger untill the player passes a trigger box? I have a trigger box but i don’t know how to set it to move once player overlaps with the trigger?