I've not added spacers on my GLC 43 but I'm sure it would look great. I did add spacers on two Porsche 911's and loved the way it looks and had zero problems. I would just make sure you bought high quality spacers from someone you trust and have used before. Please send some pics of before and after.....I'm sure it will look great.
Different people have different ideas. If we buy the high quality spacers and install them correctly, then there is no problem at all, but remember to use the spacers with hub centric, that can aviod the problem of vehicle vibration.
The quality of a wheel spacer can be determined from its materialuse, manufacturing process, hub centric and lugs.
I had BONOSS wheel spacers 20mm on my car for almost 21,000 miles, with no issues whatsoever.
Maybe you can try what I installed.
They have aluminum alloy 7075-T6 and 6061-T6 options. 7075-T6 will be more expensive but better in quality. In fact, 6065-T6 is good enough. Its tensile strength, yield strength and elongation can support our normal use. And its lugs are grade T10, studs are forged grade 12.9.