For me I think unity is all about the dev, if the dev is a good programmer unity works well, otherwise its cpu hungry, freezes a lot etc. I think its not easy to make a good unity game that is also optimized well to work without eating up your cpu/gpu.
for rpg## games, it all depends on the scripts added to the game, and yet again, if the dev is a good programmer. when I worked with rpgmv, I edited the core scripts and changed it to work as I wanted, but many devs they add scripts above the core scripts, and that is of course eating more cpu and can create bugs.
yesterday I tried Ren'Py, and it didnt start in one of my "work" laptop w7 computer.
while my own maple.x engine started without any issues and runs smoothly. my engine runs in 60-70fps (depending on monitor frequency) using direct2d.
that for me is a surprise, Ren'Py is the most stable engine so far, works well, is cpu friendly, but even so, not all computers can run it. I really want a engine that works smoothly and is very cpu/gpu friendly. I have plans to do one myself, but to make one takes a year or so and im busy with irl and my own game right now. but I really want to make a engine that is easy to use and optimized well.
I noticed that the gpu has bitmap size limits. my own gpu has 16384 bitmap size (meaning, a bitmap loaded into gpu can't be above 16384 width/height) but I also know that low-end gpu can have as low as 1024/2048 and thats small) a HD game can't start because of that. sure a good programmer can "check" this limit and change from gpu to cpu to allow low-end computer to run it. but do the game engine do this check at all? or do some engine use cpu only? like rpgxp/rpgvx, that seems to use GDI.
rpgmv is also a eating cpu bastard. even if improved from rpgvx is still not well optimized. scripting is easier but the cost is higher. you need to have a good computer to run it. my "work" laptop cant do it, its freezing a lot.
so for the OP question I say that rpgmv and unity are engine that gives me most headache.
while Ren'Py are so far the best one. rpgxp/vx works well, but also eating too much cpu for my taste and not monitor sync is making the moving/animation stuttering. and that for me is a big minus.