The alchemers span quite a bit of damage and statuses already. They're to guns what brandishes are to swords.

Sure, having another shadow variant might be nice, but alchemers already have so much going for them I really don't see too much use with 'em on a different damage type or status, as the bullets are too slow for fiends/beasts and already cover 3/4 main statuses.

If we need any additional guns what we need is another catalyzer. Right now they all have a pretty fair balance going on between each other.

I bring it up as fiends/beasts are more of a "attack faster not harder" enemy. If you attack fast enough they can't do much.

Alchemers are just a more heavy gun type. It's designed to hit hard in crowds, and the enemies that crowd up don't typically find themselves in the beast and fiend families.

And yes, I have used alchemers on beasts and fiends myself. I wouldn't recommend it but, except for shard bombs, you can really fight anything with any weapon and win.
I've used the riftlocker, and other piercing guns such as callahan, blitz, and gilded griffin. From my experience, the riftlocker is really bad at fighting fiends and beasts when compared to its alternatives (callahan's fast bullets and knockdown prevents enemies that dodge from dodging and locks them in place for a moment, blitz covers large areas, gilded griffin keeps you moving and shooting but requires more precise aim than I have), and an alchemer has an even slower bullet. Yes, you can shoot to the left of a devilite and have it dodge into your bullet for massive damage. Yes, you can shoot a wolver when they bark at the sky. Yes, you can fire charge shots into a gorgo or greaver swarm... However the alchemer just simply doesn't showcase itself all that well in those scenarios.

I'm against a piercing alchemer for several reasons. First being that it'd act like a substandard blitz/riftlocker combo. Might find some niche for chargespamming vanaduke but if you're really intent on doing that you can use a hail or storm driver.
Second, ricochets and "piercing" don't really click in my head. Is it just a technicality and not really matter? Yes, it is, but it's one of the factors and I'm not gonna let anything small get away just because it's small.
Third, slow bullets don't work great against enemies weak to piercing. In my experience, fast weapons trump slow ones for both enemies, which is why I'm so supportive of a WHB becoming piercing damage to encourage fast play. It's also why all piercing weapons until the gunner update were fast weapons - flourishes hit fast and follow their targets, callahan bullets are large and quick, blitz bullets are all over the place but travel quickly, antigua fired quickly and its bullets were quick too(same speed as blitz). Bombs aren't really about being fast but piercing bombs did have quick charge times.

To further clarify, I don't only mean slow bullets against devilites and wolvers. Chromalisks will attempt to dodge bullets as well (however futile it might be), and trojans must be fully taken advantage of during their vulnerable state (that is, attacking). Greavers will be interrupted by any attack while attacking, so it's really a "what can I hit the most enemies with the fastest" when up against them. Gorgos will still sway when idle or attacking, and thus will move out of the path of your bullet unless you give a fair amount of lead, and even then the awkward movements of the gorgos will prevent landing charge attacks on them at the critical point (right corner), and richochets will not be effective.

So no, when I say "beasts/fiends", I don't only mean "devilites and wolvers". Wolvers and devilites are the best examples to give of slow weapons being less effective, but it really does spread across the whole family.

TL;DR - beasts and fiends mean "beasts and fiends", not "devilites and wolvers". The way they work is either anything works on them or fast weapons are much better. Alchemers are slow weapons that rely on slower targets, else they rely on a good gunner who can reliably switch-shoot.
I'm against a pierce alchemer because A) it's unnecessary with current pierce guns, B) pierce + ricochet = bad times, C) slow bullets + piercing = bad times.