I have used them with little issues in the past, but do not prefer them for the reasons mentioned above. I would say that if the only location for you to tap into a wire is in such a limited space that those are the only option, then go ahead if it is your ONLY option. Just know that if you have problems it will be that much more difficult to fix than it was to install.
Otherwise do one of these options:
A) Run the power wire directly to the fuse panel. Run the ground wire to a ground distribution block. You will be much happier with the cleanliness and having it being easy to troubleshoot.
B) Cut and strip both ends of the wire you are tapping into, put heat shrink around the wires, tie in your new connection, solder, seal the heat shrink
C) Cut and strip both ends of the wire you are tapping into, put heat shrink around both the feed end of the wire and around the new and old wires, butt connector the feed side, put the wire and the old wire in the other end of the butt connector and crimp.
That is really all the options I would recommend doing. Anything else you are kind of mickey mousing it and open yourself up to problems down the road/river.