Brand loyalty is pretty strong in the tool world, because switching between tool brands is highly unsettling.
Installations is a mainly a matter of muscle memory, you see. After thousands of repetitions, a technician learns exactly how to position their wrist and how to balance their forearm and how to twist their thumbs and on and on and on. Trying to use unfamiliar tools means that all the skills they don't even know they have are now useless and needs to be relearned, leading to stress, panic, and confusion.
You'll find this phenomenon in most activities that rely heavily on muscle memory. Chefs are loyal to a brand of knives, golfers are loyal to a brand of clubs, gamers are loyal to a brand of keyboards, and so forth. If you ask them why, most of them will struggle to explain themselves or will invent ridiculous post hoc explanations, but the actual reason is muscle memory.
This holds even when switching between tools. Many technicians, if given a choice, will prefer to source all their tools from the same manufacturer. Toolmakers, like knife makers or golf club designers or computer keyboard builders, tend to use a consistent design language between products, meaning that a technician who is comfortable with a Milwaukee drill will very likely feel comfortable with a different Milwaukee product from the very first time they pick it up. It will look and feel and act familiar, which is very psychologically important to a technician. This will lead to very fast adaptation of the new tool, allowing them to use it to its fullest potential almost from the first day on the job site.
The difference between Milwaukee tools and Ryobi tools is particularly egregious. As Karas says, Ryobi is a consumer grade brand, and Milwaukee is a professional grade brand. Not only will the tools feel and act very different, they aren't designed to stand up to daily use, and will probably need replacing before the year is out. No wonder your techs are feeling mutinous, although it sounds like they aren't smart enough or articulate enough to explain all this.
Next time you have to buy new tools, get the buy-in of the people that have to use them. They, of course, will always want the fanciest and most expensive option, because it's not their money, so don't actually feel obligated to follow all their suggestions. But at least let them feel heard. And if you want to give a Christmas gift to your computer gaming nerd nephew, give them a gift card instead of trying to guess what keyboard or joystick they want.