Letting anyone seal over old silicone
New silicone does not bond to old silicone, and mould trapped underneath grows straight through the fresh bead. If the technician is not spending most of the job on removal and drying, the reseal will fail within months. Ask to see the joint clean and dry before the new bead goes in.
Putting a full-motion TV arm into bare plasterboard
An articulating arm multiplies the TV's weight into leverage at a few fixing points, and plastic plugs in gypsum cannot resist it. The mount holds during the demo and fails weeks later. Insist on metal studs, a backing board, or a different wall.
Judging the quote instead of the method
The cheapest quote often prices a different job: silicone over silicone, plugs instead of proper anchors, no anti-tip strap on the wardrobe. When quotes differ widely, ask each vendor how they will do the job. The one who mentions wall type, anchors and cure times without prompting is usually the one worth paying.