As noted by @Jonathjon are a few have metal struts - I may make some out of 3mm aluminium eventually, but my plan currently is to use ACM.
I don’t think there’s any evidence to suggest that metal plates or struts produce a more rigid build, but they look the part!
Sorry about the finger prints in the pic … if you want to go nuts. 6mm aluminium. These were laser cut and while the holes are accurately placed, they did have some anomalies in shape and direction (particularly the 3mm ones)
Full build thread here, but it’s a very long and distracted read and it’s not finished yet!