E-juice is a wonderful thing if you vape, considering that it is in fact what you’re vaping. However, there are certain blends that just tend to gunk up your coils and can otherwise damage your tank in some way. The main concern here is the type of e-juice that you’re using, because some have more of an effect on your coils than others. Beyond that, it is a question of your coils and wicking materials, the type of atomizer that you’re using, and even the device itself.
The fact is that some e-liquids tend to gunk up your coils faster than others, and while the ratio of PG to VG has an effect, it isn’t the only thing. Some e-juices just have a habit of creating more gunk on your coils based on the flavors themselves - in fact, there’s a direct correlation between the shade of an e-juice and the amount of residue that it leaves behind. Different flavors are made from different ingredients, and some of these do leave behind resins and residues more than others.
The ratio of PG to VG is an important factor as well - because VG is denser and slightly sticky, it can leave behind a lot of residue. In general, e-juice made for cloud chasing has about a 30/70 ratio of PG to VG - which is what cloud chasers want because it produces a lot of vapor. However, this can gunk up your tank faster than a 50/50 or a 70/30 PG to VG ratio would. Different vapers like different things, and while PG will give you better flavor it will also give you less vapor - so if you just vape or happen to be a flavor chaser then you might not need to worry so much about your tank getting a bunch of residue in it as you would if you’re chasing clouds. The interesting thing here is that there are a few e-liquids out there which crack plastic tanks, and you should be a bit careful of those.
Your device itself will influence all of this. Depending on your tank, you may face a lot of problems or virtually none - if you’re using an RDA and regularly change your wicks and clean your coils, then it’ll probably come as a surprise that an e-juice could damage your coils or tank. RDAs might be fussy and require a bit of know-how to use, but when it’s breeze to clean them and you’re probably quite used to burning all the crud from your coil when you replace your wick. And if things get bad enough, you just replace the coil. RTAs aren’t a whole lot different in this regard either, with the worst-case scenario being that you must replace a coil that costs a few cents and five minutes of your time. Sub-ohm tanks are a different story because you’ll have to replace the whole coil unit, and the unit itself isn’t so easy to clean or dry fire - but it can sometimes be done.
Ultimately, it’s a trade-off because you must choose between an easy-to-clean RDA that requires a bit of skill to build, or a tank that’s harder to clean but requires less skill to use safely. And plastic tanks can get cracked sometimes by a given e-juice, unlike a stainless steel or glass one. Undoubtedly, a cartomizer will suffer the most from a juice, because it’s a pain to access the coils or clean the tank. Considering that cartomizers use a small coil with high resistance, it’s almost impossible to clean these out properly or replace the wick when it gets fouled - meaning that these devices are the most vulnerable to an e-liquid and the most difficult to deal with. You’re basically looking at getting a new cartomizer instead of cleaning. And because some e-juices crack plastic tanks, you can bet that your device is also vulnerable this way.