It is a common myth that Channels 8-14 are limited to 1/2 watt, narrowband, because they are "interstitial" between repeater input channels.
The fact of the matter is that historically before the 2017 Reorganization, Channels 8-14 were FRS only and not accessible by a GMRS licensee, except for those GMRS licensees who used "combo" radios that allowed both FRS and GMRS or strict FRS radios, and in essence when using Channels 8-14, they were using them under FRS rules and not GMRS rules.
The restrictions on Channels 8-14 were instituted to keep the Family Radio Service from interfering with each other. Those restrictions were carried forward in the 2017 Reorganization.
Channels 1-7 are also "interstitial" between repeater output channels, yet allowed to transmit up to 5 watts on GMRS.
The opportunities for interference to repeaters is a factor not only of power and frequency proximity, but also geographical proximity.