Mismatched chain and chainring
An 11-speed chain will mesh with and be driven by a ring designated 8-speed without issue, barring corner case exceptions. (There's nothing necessarily stopping manufacturers from cheating or being sloppy with keeping everything that's nominally 3/32 cross-compatible even with parts of disparate eras/types as in this example, but it's not an issue one sees too much of.)

If you did have issues shifting, it could be a combination of chain outer width, FD travel, side plate profiling being a poor match, and/or the chain's potential ability to get stuck between rings, which I have seen albeit not often.

One confusing thing about this topic is that chainring spacing is one of the least standardized things on bikes, it varies a little but not much between speed generations, the numbers are close to one another but not identical for how each manufacturer does things for different speed generations, and what actually determines the spacing is a combination of ring thickness, spider tab thickness, tooth offset, and presence or absence of spacers, so non-stock chainrings being in the mix can easily affect the actual spacing.

Usually combinations like what you propose can be made to work. Often they work well. But it's never a guarantee they work, or work without compromised shifting performance. 

Part of what makes the issue murky is that in terms of professional shop practice, writing up a work order to create this kind of setup for a customer as a repair, build, or upgrade is pretty tenuous at best. In that context, setting yourself up to mess with something finicky that might not even work or be highly compromised is a great way of wasting time and creating a complicated situation. So bike shops and pro mechanics have good reason to take a different baseline attitude on the topic than a home mechanic.

Presuming we're talking about road, in the case of 11-speed, part of what you're up against here is that 11-speed front derailers often act like they barely have enough total travel even when everything is as the manufacturer intended, particularly Shimano ones. It would in no case be surprising to me if this were enough to keep what you're proposing from working. It may also be able to work fine, or you may be able to hack the cable anchor position for more travel.

Putting a narrower chain in a wider front derailer cage usually works extremely badly, especially multiple width generations apart. The usual problem (for a double) is that by the time you've gotten the low limit adjusted so the inner cage is close enough to the chain to get a good shift off the small ring, it wants to have way too much gap with the outer cage once you're on the big ring, and chain drop and poor downshifting result. Triples compound the problem. This can potentially be mitigated by manipulating (squeezing closed) the cage. When it's multiple width generations like this I personally just don't do it, even on my own bikes.  If you decide to it's going to be an experiment where you're completely out your FD if it doesn't work.

One thing you see parroted on the internet is a "a front derailer is a front derailer," as in they have a great degree of interchangeability, which is just not reality. Cage width, profile, actuation ratio, and in the case of triples minimum mid-to-high tooth difference spec are all critical.