I've had an Opus BT-C3100 V2.2 since 2016. It's been great, but now one or two of the slots are acting all buggy. At times I'll get a null reading on cells that are not dead. Other times Cells will never finish charging even though it reads the current and they get plenty hot (gotta imagine it's intermittently draining the cells while charging when that happens). Any one else had similar experiences? It's grown unreliable to the point of being replaced.
Sorry to hear that. I had one issue when I first started using mine (about the time you got yours), but it's by design, and I've had no other new / flakey issues. Although I consider it cheap junk (even though there's nothing like it on the market that's likely any better in that regard, IMO), it's been stable with no new symptoms and generally does what it does OK. That said, mine has had a pretty easy life.
The issue I had (which could
possibly be involved and would likely get worse with time if not mitigated) is chronically poor connections due to insufficient slider terminal spring tension (in addition to probably crummy contacts / plating) using shorter cells. This requires me to use a dummy / shorting cell together with 16340s to get adequately reliable connections, and that caused all sorts of flakey symptoms until I realized the deficiency. Having to use those is a bit of a pain, but has eliminated the problem(s), so I get by. That's somewhat less likely to be a problem if you're having difficulties with longer (18650) cells, although with time / use it could. If in doubt, as a long shot you might want to clean those sliders and terminals with some good stuff. I used some DeOxit D100 on my sliders etc. about a month ago (don't know what took me so long), and besides just being good practice, I believe it has stabilized my 'internal resistance' measurements somewhat (Quick Test).
No other ideas here. The only upside to cheap junk is that it's cheap, and thus doesn't cost too much to replace. If you decide to replace yours and find something you believe is 'better', let us know, however based on what I've seen / read, I don't think you'll find anything significantly better quality on the market currently. ICBW though, so let us know if you do!
EDIT: It just occurred to me that I should note that my experience(s) with my Opus are entirely with Li-ion cells, so I can't speak to most of what this thread seems to be focused on (NiMH). That said, null indications, connection issues, etc. to which I refer are of course chemistry-independent.