'Functional training' is a hodge-podge of confused ideas and fads, mixed in with a little bit of truth (the little bit of truth is that at some point, some amount of sports-specific work will be very helpful).
A lot of 'functional training' is just stuff with unstable surfaces and awkward loads. Unstable surface training is good for training balance, for getting better at performing on unstable surfaces and for some kinds of rehab, and that's it. Awkward loads can be a good part of a training programme, if your sport involves moving awkward loads... but the best way to build your ability to move them is to mainly practice with a non-awkward load (i.e. a barbell) and then sometimes practice with the awkward load too.
Some 'functional training' is just HIIT or circuits. That's fine- if you need to do develop your anaerobic system more than your sport is already doing, then you should do that kind of thing, with sports-specific movements. But that's just one piece of the conditioning puzzle. For a lot of people low-intensity work plus their sport should be enough.
For some people doing 'functional training' means doing bodyweight exercises instead of barbell work. It's true that advanced bodyweight exercises require a lot of strength (because you are in a mechanically disadvantaged position). It's also true that being able to exert strength from mechanically disadvantaged positions might be relevant for some sports. But.. advanced bodyweight exercises (ie gymnastics) are technically very difficult to do and need more equipment. Easier to learn and do the barbell lifts. And barbell lifts are much better for lower body strength.
So for people who are just starting out, it's really better to just work on the athletic basics: build strength in the basic lifts, build the aerobic base, get basic flexibility and mobility. Once those are there you can add some sports-specific stuff, e.g. selecting some main movements that are more similar to the movement of your sport, doing some stuff with awkward loads, adding HIIT with movements that resemble your sport, adding 'conversion to power' stuff like speed work and plyos. I've never really seen a whole programme written out for something like that, unfortunately. I think that stuff like this is generally done by proper S&C coaches, not internet celebrities, and they don't give that stuff out for free.
This is just my two Dinar. Feel free to ignore me.
