In a typical 3-of-5 setup, we use three different hardware/software platforms at one time - currently Trezor, Ledger, and a mobile phone (either iOS or Android). Neither software nor hardware wallets are perfect. From time to time a vulnerability is discovered in one brand. Relying on multiple brands and types of wallets prevents a vulnerability in one brand from compromising the overall storage system.
This also protects against supply chain attacks. If a malicious actor were to infiltrate and tamper with the devices produced by any one supplier, it would not be enough to compromise our clients’ funds.