Having all your limbs amethyst really causes more problems than it's worth. For one, because of the extreme lack of flexibility your DEX and AGI will be horribly low, whilst your LStr and AStr will be very high, as well as limb health.
The problem is this: you will have a very low chance of dodging let alone landing a blow (although should you do so it will be devastating!), and although your limbs will be able to survive many hits it is important to remember that your other body parts cannot be changed. That means that all your vital body parts remain with very little HP, and if you remember that your torso takes the most damage on average, that means that not being able to dodge an attack will lead to a quick death.
My strategy for artificial limbs is this:
Should you encounter a SoCM and want to use it for changing a limb, generally the best materials to choose from are from the cloth/leather branch.
This would be, in order of scroll of harden material ranks: Leather, [Nymph Hair], Hardened Leather, Troll Hide, Ommel Hair, Phoenix Feather, Dragon Hide.
The best over all is Golden Eagle Feather, but that cannot be obtained through harden material scrolls. Note that as you go up the ranks each requires a higher INT to harden.
My advice to you is that if you should encounter a SoCM very early in the game (e.g. in the underwater tunnel) the first material you should transform your limb into is troll hide, and then later harden it to ommel hair which is only difficult if you didn't start with 10 INT or more. From there you need to read, pray and
science talk to boost your INT to 20 in order to harder your limb(s) to Phoenix Feather. Phoenix Feather is
very strong in both strength and speed, for example in one run through the game I had a character with two Phoenix Feather arms and he plowed through everything that came at him (including the shop guards and shop keeper in the gloomy cave AND most of the dwarven city populous in LIVAN) but was finally taken down by a couple of veteran kamikaze dwarves.
And that is the final danger of artificial limbs: They raise the danger level to the point that very strong monsters will come at you earlier than they would normally, and you are likely not to have good enough equipment to withstand them. So it is important to monitor your equipment and adjust your limbs accordingly. In short, if you are still wearing iron boots and an ommel hair armor piece, don't go power hungry and turn your limbs to dragon hide.