If you want to use incendiary/cryoliquid pistols, I would say Mad Chemist is pretty much a must since the On Hit effects of chemical pistols are their main selling point.
It increases the duration of freeze to 2 turns instead of 1 for cryliquid pistols and doubles the already high secondary damage over time for incendiary pistols.
If you want to use acid blob Mad Chemist is good as well, but not nearly as mandatory. It doubles the secondary damage just like with incendiary pistols but it's not going to be as high.
Another reason why Mad Chemist is not mandatory for acid blob build is that (at least in my opinion) you should acquire XAL-001 unique acid pistol, Chemical Assault Unit Armor and craft infused siphoner leather tabis as soon as possible.
XAL-001 deals more base damage than any other acid blob pistol you could possibly craft and more importantly it costs mere 16 AP to fire instead of the usual 25. The On Hit effect is slightly worse than that of endgame crafted acid blob pistols, especially with Mad Chemist, but 9 AP less to fire more than makes up to it.
Wearing CAU armor and infused siphoner leather tabis makes you almost immune to acid (about 90%/35 with both) and immune to immobilization so you don't need to worry about the gun leaking acid at you. In fact it's going to be beneficial as you will be spraying acid puddles all around you that can damage enemies.
This build goes very well with toxic gas grenades too, both thematically and thanks to CAU armor 20 bio DT and acidic entanglement. With grenadier the cooldown of gas grenades is only 1 turn, so you can fill up entire rooms with gas very quickly and make enemies stay inside them thanks to acidic entanglement. You can safely stay inside the cloud yourself as long as you don't stay in the tiles with the highest concentration of the gas. I guess you could risk staying even in tiles with high concentration if you wore a gas mask in addition to CAU armor for a total of 37 DT against bio damage.
