From what I know there is nothing dangeriously wrong with the practice of girth hitching slings. Below are a few points that may help to consider.
These can be very difficult to untie if heavily weighted due to a fall, extended top-roping or a hanging belay.
Although a girth hitch will do in many situations it may be be faster, but harder to equalize when setting up more questionable anchors with natural pro. One of the many options is to use a cordellette instead of multiple slings or the climbing rope.
Also if you look at the strength rating of knots (see "Freedom of the Hills" by the Mountaineers) a girth hitch is a bit lower than water knots and fishermans...so depending on the sling type this can lower the strength of an anchor.
Hope this helps,
Justin
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Justinuncanny