5GHz Bridges absolutely need to have a clear line of site. With 2.4GHz Bridges, a clear line of site is still the absolute best, but it is a little more forgiving with line of site issues. You may be able to get away with a few trees in the way, or a few power lines.
2.4GHz may have more issues with outside interference from other devices that operate on the 2.4GHz Frequency (Cordless Phone, Wireless Routers, Baby Monitors, etc.), because there are just more 2.4GHz devices being used in the market.
5GHz will be less likely to have problems with interference with other devices because there just aren’t as many 5GHz wireless devices deployed. Both 2.4 GHz and 5 GHz operate in the unlicensed frequency spectrum.