They're not so much unreliable as ... really not very sensitive, as I understand it. Viral load in covid sufferers follows a curve with a rapid ramp up and a slower ramp down, both PCR and lateral-flow devices require a certain minimum level in order to read positive. PCR is much more sensitive (because it's an amplification technology, so can "see" a lower level of viral load) than lateral-flow devices - which means you can be "safe" on an LFD and "unsafe" on a PCR for a small amount of time on the ramp up and a fairly large amount of time on the ramp down. The level at which you're at risk of passing it to other humans is I _believe_ somewhere between the two tests, so it sounds like you maybe got unlucky with an LFD or two, but the results you report are about what I'd expect :(LFDs are decent for telling you if you're infectious _right now_ - they're much worse than PCRs for telling you if you have covid at all.
Disclaimer: I'm a software engineer, do not take medical advice from me. My partner is a biochemist and I regularly chat with a couple of council covid-response officers, and the above is the explanation I've had from them when I've asked.
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