There isn't really a one size fits all to this. There are people who have amazing results with simple visualisation or sigils and then there are those who have powerful experiences with the most complex grimoire / traditional rituals. I don't think anyone can really reduce effectiveness to simplicity or complexity in magick because in the end, it is about how the individual vibes with what they are doing. There are people who thrive in simplicity and will be better suited to get results with those types of practices. Then there are people who require 100 steps in a ritual, with every correspondence accounted for, every word memorised and every piece of theory absorbed.
For me, I tend more to the complex side of things, but recently I have learned to embrace simplicity in magical practice more, especially as my mundane life has become more intense. I always enjoyed having a framework to bend something around and I always preferred having a "theory" as to why something works in magick. So things like evocation and spirit based magick work nicely for me because there are entire systems built around such things, with all the theory and framework I could ever need in order to tweak my magick. But as I said, I learned that powerful results can be achieved with simpler practices than evocation such as mantric magick or verbal spells, mixed with powerful intention and focus. These forms of magick have recently produced great effects for me and, being perfectly honest, I know very little about those spells in terms of theory and background. They are like black boxes to me and I have to force myself to accept that fact, but yet they work without all the complexity. So in this sense, I see both sides of the equation now.
But I think the key lies in the individual and how capable they are of bringing focus, clarity, energy, and intention into the ritual. Something done with powerful concentration, clear intention, and a strong desire will be more effective than the opposite in any ritual, simple or complex. Things like meditation, energy training, and internal work are always going to help the inner dimension of magick, which powers the outer dimension of it. If anything, one could argue that complex ritual is partly a way to bring focus, clarity, and energy into intention in order to produce a magical effect. If you look at it like that, then simple rituals are actually the harder ones to carry out effectively than complex.
But I digress. Both have their uses and I use both in my eclectic way of working. I tend to use simpler rituals for simple intentions and complex rituals for more complex results. I think this sort of analogous way of doing things is quite effective (though people feel free to disagree). A simple practice, such as mantric magick or visusalisation, would require an immense amount of focus in order to be used with a complex intention. A complex ritual, such an evocation based one, gives you a lot more surface and layers to express a complex intention (e.g. think a pact). But a complex ritual with a simple intention seems almost like waste of energy in some cases. But again, people can vary on this.
For me, taking an engineering perspective is a nice way to look at this. There isn't a point in debating simple vs complex, unless you want to debate it. For real results and effectiveness, use the tool that works for your situation. Test and experiment to see which ones work for you and refine your approach.