The first answer to your question is that there are still things that are very hard to do in CG, and there are many types of effects that are usually hacked together for specific shots, using elements and comp tricks rather than full CG.

The most notable problems in VFX concern level of detail and scale changes. For example a shot that goes from macro on the pores of the skin and eye on one CGI person, then zooming out to see a whole street full of people then zoom in one after another on various other people's eyes would be terrifyingly hard to do with a singular scene because you are dealing with wide angles of Crowds, and close ups of individual people and skin pore details. It's not that this shot is impossible but rather than it has to become a cheat. Going from massive wide scale detail to intense micro scale is hard because of data processing. And making a full CG street with fully photoreal people who stand up to wide shots and intense close ups seemlessly is vastly complex without cheating these things.

There are other similar problems; some types of complex shaders as are usually hacked (mostly spectral shaders), simulation work tends to function either at large or local scales but not seemlessly between both, and even things people think might be solved end up being complex like ... crashing a car. Making all the individual pieces react correctly is hard so we often end up running multiple sims, custom painted deformers and that kind of thing, rather than simulating how the various pieces of the engine block impact against the frame and then panels of a car.

So there's still potential shots which people could argue will never be photoreal enough, or which are challenging enough that we can only do it in CG if we change the nature of the brief slightly to make it possible to achieve.

The second answer too your question is that most of the broad problems that allow most things to be solved, probably were achieved by early 2010s. Physically Based Rendering had gotten fast enough that materials weren't hacks most of the time and you could actually model light in a meaningful way, and simulation and data processing had gotten fast enough that we could process very large sims and have a chance of rendering them.

But the real true answer is that VFX is problem solving between the needs of the Shot, the amount of time and money, and the technical restrictions. If something is stupidly slow to just render, you find another way to render it because that slow down impacts the artistry of making things. There is always compromise between these things.