The cool down time and ringing of cylindrical metal piers I've used are non-issues (10" to 12" diameter pier < 40" tall). The latter would generally be of concern if one was prone to constantly rapping on a pier during imaging or visual work, but I suppose it could come into play if there were strong wind gusts hitting a long OTA (a non-issue in an observatory). I've used short and long OTAs on quality metal portable and permanent piers and never had a ringing issue.
I haven't studied the effects of torsional loads on an X-bar, but I also haven't seen any such problem with decent cylindrical metal piers.
Can you please point to a FEA of X-bars versus cylindrical tubes? I've used them in my lab to support micro-manipulators that work in conjunction with microscopes. I hadn't thought about using them for astronomy pier apps.
I am not aware of anyone doing any serious FEA simulations (Finite Element Analysis) of amateur-class observatory piers but there is certainly a lot of FEA that has been done in and around larger professional observatories. The results would be interesting if someone wanted to go through all of the work to create them.
An "X pier" has more surface area, no dead air spaces and a much-higher surface-area to mass ratio than a tubular or rectangular pier. This isn't likely to matter much in a lab but in an environment where there are temperature swings and you want rapid thermal stabilization, it has advantages.
Unlike research-grade microscopes or precision mechanical laboratory stages, telescope mounts carry heavy, relatively-powerful, active motor assemblies and additional components that are vulnerable to unpredictable external influences. The real challenge for telescope mounts and piers are resonant frequencies and effective damping. In the bad-old-days, this was usually solved for telescopes by lots and lots of mass (more or less.) Nowadays FEA, advanced materials and significant fundamental research have given us much-better tools to work with.
A tubular steel pier does not have near the diagonal rigidity of a steel X pier. An FEA comparison of the two would certainly show that all other things being equal, the X pier would be much more resistant to harmonic vibrations created by wind or the mount's pulsing motors running at different speeds.
And while steel is elastic and concrete effectively is not, steel has a high thermal conductivity that has benefits in an observatory when used carefully.
In "The Design and Construction of large Optical Telescopes, © Springer 2010", Section 6 discusses telescope mount mechanical issues at length, including the challenges and limitations of FEA simulations in the design of precision telescope support structures (mount, base & foundation.) It is an excellent reference resource for anyone desiring to build a serious, effective amateur observatory. Be warned that it is filled with math.
I hope this helps.