Is it safe to mix two RAM kits instead of buying a matched set?

Usually yes, but not always. Two matched kits of the same generation will likely work, but they’ll run at the slowest kit’s speed. The risk is instability from different timings or voltages, especially at high frequencies. Intel boards are more forgiving than Ryzen. If building from scratch, buy a matched kit. If upgrading and you can accept a small chance of troubleshooting, mixing is fine.