Is mixing RAM kits likely to cause a boot failure?

It’s a gamble, not a guaranteed failure. Two kits, even with the same specs, weren’t tested together, so the motherboard may struggle to find common timings, leading to a black screen or boot loops. It can work fine at the slower speed, but the risk is higher than a matched kit. If you’re okay with troubleshooting and possible returns, go for it; otherwise, spend the extra $20 on a matched dual-channel kit.