It all depends on your application...

They are really useful for GDI engines to minimize the effect of carbon build-up on the intake valves. They do not completely fix the issue. GDI's are prone to carbon build-up because gasoline does not "wash" over the intake valves compared to port injection. However, the PCV systems on modern GDI engines have been refined to minimize carbon build-up, and most nowadays even have a factory oil separator built into the PCV system. But having an oil catch-can installed does not mean you should skip out on walnut blasting your intake valves. Keep in mind that installing an oil catch-can may be illegal depending on your local/state emissions laws/regulations.

I don't see them being useful on port injected engines. They do not suffer from carbon build-up because the gasoline is injected before the intake valve, cleaning any oil residue off of them before entering the combustion chamber. So basically, the port injection naturally takes care of the carbon build-up for you, whereas on GDI engines, you have to take care of it yourself every 60-80k miles or so. 

My opinion on the cans is this:

If you have a gasoline direct injection ONLY....then it is a good idea. Port injection allows gasoline to wash over the intake valves, which lets the detergents in the gasoline eat away any build up and keep them clean. Even if you have direct injection but also have port injection, your valves are still getting cleaned and I really do not see the point. But if you have a direct injection only engine, the gas is injected directly and does not wash over the valves, so that is where you get build up.

Here is a good example: my 2020 Accord Sport 2.0T is direct injection only. So the first thing I did was put a catch can on it. Now, you only need one on the passenger side. The driver's side is your "clean side" and does not need a can. On my car, which is 100% stock, I am catching about 1/4 cup of oil every 2,500 miles under regular driving conditions. I also have the known Honda oil dilution issue, so the fluid smells strongly of gasoline but that's another issue.

Excellent video by Engineering Explained and at 7:46 goes over the different catch cans available and features of each (from cheap to expensive).

https://m.youtube.com/watch?v=T-B4VRxAtbw