I don't see why it would be bad to use a ferule to join wires.
From my experience when using a good crimping tool these things are pretty much impossible to get off again. When crimping connector pins onto wires i always test the first one by grabbing it with pliers, wrapping the wire around my finger once and pulling apart as hard as i can for one or two seconds. If the wire starts slipping from my grip instead of pulling out i call that a successful crimp (For tiny non power wires i do a similar test but pull much less since otherwise id tear the wire itself in half)
Its important to test the first crimp you make with a given tool and wire combination. If you use the wrong setting or wrong die on your crimping tool you might end up with something that looks crimped perfectly fine, but will pull out with relatively little force. Sure your setting might match the ferule/crimp you got, but if the wire is not filling the whole ferule/crimp then it won't grip it inside.
If there is more force than that being exerted on your wires then you have installed your wiring harness wrong.
Also you can remove the plastic part of the ferule by pushing the metal part towards the plastic part and then grabbing the plastic with pliers and ripping it off. This lets you make nice low profile crimps for when space is important. If you are worried about pullout then put both wires into one side, this makes the pulling force sideways when you pull the wires apart, so the wire itself will always break before the ferule breaks.
I have used ferules for joining before, but yeah its not what ferules are intended for. The correct thing for splicing wires are these crimp butt splices:
https://uk.rs-online.com/web/p/crimp-butt-splice-terminals/0534288A/As a bonus they also are already insulated so no heatshrink needed.