With these complexes it seems that the idea is to use a relatively light weight and keep the movement going for a long time
I think this is one kind of complex or circuit, but it is definitely not the only one. I think it is common on youtube and instagram because light weight makes someone look more competent than they actually may be and it is popular among a certain large segment of the population to want light weight for long durations. I suspect the latter is why you see long circuits, and the former is why you see light weights. Of course, they could be using light weights to demonstrate impeccable form, and as it is a demo and not a workout that may or may not be relevant...
This is opposite to the other style of training, just doing one exercise in sets, ladders or other woth the aim of getting strong.
I don't think these are necessarily opposites, or at least "the aim of getting strong" does not necessitate them to be different. As an example, you could look at something like ROP (clean and press + pull-up ladders), or something like Brian Alsruhe's programming in giant sets. Both of those examples are to get strong, both use circuits (or circuit-like entities).
What do people here think of complexes?
They are a valid way of training, but like everything they are a tool that must be used appropriately.
They are a tool, so it depends on what your goal is and what you enjoy to do. I think the enjoyment part is more important as proper design can make the circuit work for almost any goal. If I love circuits, I will use them and enjoy them, sticking with my training and seeing great results; if I have a client who hates them and I program them for them, they are less likely to adhere to the program and less likely to get results. The "key" here is not the circuit, but in finding something that "works" for the individual.
Should they be kept to a number of movements?
I'm not sure, but you cannot sacrifice your attention in the moment to focus on the next thing (e.g. I'm not in the moment focusing on my cleans because I'm thinking - is it the getup that comes next, or is it a front squat?). I'm not sure where superset - giant set - circuit/complex/chain begins and ends. I really love pairs of movements - cleans and presses, presses and front squats, swings and cleans, front squats + farmers walks, cleans and front squats, etc. Having a couple pairs in a session hits a lot of big ticket items. A "fundamentals" circuit is fun too - e.g. swing, clean, press, squat, snatch, get-up.
Yes. Because they are difficult. No... because they're difficult.
And anything else that you care to comment
There are so many different ways to use circuits. It isn't just light weight, it isn't just long durations.
I'm really enjoying "slow circuits" - for example, 10 swings, rest 30s, 5 presses, rest 30s, 5 squats, rest 30s, 5 cleans, rest 1-2 minutes, repeat. This is definitely not the same as doing all of them without rest. You aren't restricted to relatively light weight and the quality of the reps is A+. I mention it because I've started doing it and I enjoy it, and it is sort of "in-between" the two things you've mentioned.
I think they're a great tool when training a client or a group of clients.
@Boris Bachmann hit the "worst" part of circuits - sloppiness. If you start getting sloppy, increase rest time and/or decrease weight - or stop the training session. Don't get sloppy, and stay focused on what you're doing
now.
Daggum I guess I felt talkative today...