LED strip has one of the higher return rates in the category, and the cause is rarely the LEDs.
First problem: voltage drop. A 12V strip loses visible brightness past about five metres. A 24V strip holds to roughly ten. The mechanism is simple, copper has resistance, and the further the current travels the more it drops. A five-metre 12V 5050 strip at full white can be 15 to 20 percent dimmer at the far end. You see it as a gradient tail.
Three ways around it: use 24V, feed the run from both ends or in sections, or specify constant-current strip. CC strip carries constant-current ICs along the run, holds brightness over distance, costs more, and simplifies installation.
Second problem: sealing. IP20 is bare board for dry interiors. IP65 is usually a surface coating or a sleeve for splash protection. IP67 and IP68 use silicone tube plus potting and tolerate brief immersion.
A common trap here: marked IP65 but only sleeved, with open ends. Water enters at the end cap, travels along the inside of the tube, and shorts the whole run. Ask about the sealing process, not just the number.
Third problem, and the least discussed: the adhesive backing. 3M VHB holds well on clean, flat aluminium. On rough plaster, dusty ceilings or damp tile it lifts within six months. That is not a strip defect. It is a mounting surface problem.
The engineering answer is an aluminium channel. The strip goes into the channel and the channel is fixed to the structure. The channel handles heat as well as mounting, and long runs depend on it for life.
Other parameters worth confirming: LEDs per metre, watts per metre, CRI, and whether the reel is single-bin. Colour inconsistency within one reel is the classic strip complaint, so put single-bin in the contract.
We run strip at full load for 24 hours to watch voltage drop and heat, and cut a section for an immersion test. These are cheap tests that prevent a whole batch going back.