What are the Advantages and Disadvantages of ISM Band Frequencies?

The FCC allocates different frequencies for different purposes. In the US the 900, 2400 and 5000 MHz frequency bands are set aside by the FCC for unlicensed Industrial, Scientific and Medical (ISM) applications. The lack of licensing requirements has greatly encouraged the growth of the wireless industry. These bands are used for consumer and commercial WiFi and WLAN applications as well as for commercial Radio Frequency Identification (RFID) and Supervisory Control and Data Acquisition (SCADA) applications.
900 MHz
The 900 MHz ISM band is very narrow and this limits the maximum data rates. Typically applications such as SCADA and RFID use the 900 MHz band as the data rate requirements are lower than applications found in the 2.4-5 GHz frequency bands. Many times the type of data packets being sent in these types of applications is a simple on /off command to something like a motor or value. The 900 MHz frequency is better than 2.4 GHz frequency when obstructions such as trees and leaves are in the Line of Sight (LOS). The 2.4 GHz frequency is absorbed by water found in trees and leaves which causes path loss of the 2.4 GHz transmission. 900 MHz is often used in Non-Line-Of-Sight (NLOS) applications.
2.4 GHz
For the home user and commercial business 2.4 GHz is the primary band one uses for WiFi, Bluetooth, cordless phone, printer, keyboard, mouse and gaming controller applications. Voice, video and data communications are typically used in 2.4 GHz systems requiring higher data rates (up to 300 Mbps for 802.11n applications). 2.4 GHz is the most widely used frequency and in some cases may be overcrowded. Examples of 2.4 GHz devices include microwave ovens, baby monitors, cordless phones etc. When too much overcrowding occurs, your WiFi network signal may be weak or not work at all. In some cases it's best to use 5 GHz backhaul links to connect 2.4 GHz WiFi networks as 5 GHz is a less crowded frequency.
5 GHz
The 5 GHz frequency is often used in commercial WiFi applications. Many times it is used as a backhaul link connecting two 2.4 GHz systems over some distance. 5 GHz is also the frequency used for the emerging standard 802.11ac which will provide up to 1.3 Gbps of wireless data throughput. Additionally 802.11n can use the 5 GHz frequency.