Not much to add that hasn't already been stated. I had one of the original Innokin MVP's which had a Lipo battery. My sister has it as a backup to her iStick 30, even though its now over three years old.
I've always been a fan of regulated mods that use replaceable IMR/INR batteries. A mod with an internal Lipo battery is useless if you are caught out with a drained battery; with a mod with removable IMR/INR batteries you can carry a spare or two with you, quickly change out batteries, and continue vaping.
I also like being able to specifically choose what battery to use in a mod; unlike Eskie, I don't trust Chinese manufacturers to know the best battery to put inside a mod. As an example, Joytech included an ICR chemistry battery (the Samsung 26F) with their original eVic. This was a poor battery choice considering the safer chemistry IMR/INR batteries that were available at the time.
Do not use the Samsung 26F battery above 5A or 20W!
Lipo batteries, like ICR batteries, are not "safer chemistry", like IMR/INR batteries are; Lipo's depend upon the protection circuitry of the processor to be safe enough to be used for vaping. That's probably why you don't see many mech/unregulated mods using Lipo batteries. Issues with Lipo's are relatively rare, but when they do occur they are rather spectacular in a bad way. Issues with IMR/INR batteries in regulated mods are even rarer, and when they occur they generally just vent hot gas and don't flame out of control.
I'm not a big fan of USB onboard charging, which internal battery mods use. Just my personal observations from comments on this forum over the years, they tend to have saftey issues more often than external box chargers. Again, I like to choose the charger that I'll use to charge my batteries, and not depend upon a Chinese manufacturer to make that decision for me.
Since some manufacturers are now offering replacable Lipo batteries in their mods, I'm beginning to relax my bias against Lipo batteries a little. For my

I still find IMR/INR chemistry batteries to be preferable.
Deeper Understanding of Mod Batteries Part 1
- For those who want to learn the differences between IMR, IMR/hybrid, ICR, and LiPo batteries. What do those numbers and letters on batteries mean? What's an amp rating and why is it more important than the mAh rating when choosing a battery for vaping?