My homebred version worked like a monster mount with construct traits, but "controlled", it needs a rider or pilot to controll, and this could suffer necrotic or poisonous damage, disease, mind-affecting effects and other things true constructs are totally inmunse.
We should remember "magictek" is not only for the PCs but also could be used for the DM to create special bosses. Then how can we calculate the challenging rating? Maybe the pilot is only one hit dice, but the mecha is too powerful. Do you remember D.Va (Overwatch), Titanfall, B.R.U.T.E in Fortnite: Battle Royal or Jackpot, the final boss of the Borderlands 3 DLC: Moxxi's Heist of the Handsome Jackpot?