Is there a disadvantage to keep my MacBook and Mac mini plugged in?

The newest Macs are very energy efficient, so there is little to gain by shutting them down, and there is little reason to.

My daily driver, a MacBook Air, is always on and usually connected to power. It doesn't usually get restarted unless an OS update or upgrade has been applied. If the OS or some other app were to misbehave then that would also prompt a reboot. Otherwise it could go weeks without.

I also have a 2013 iMac that remains on 24/7 as a media server.

I've never had problems with either of these, nor with many, many other Macs over the decades.

Here's some light reading from Apple regarding battery health in the MacBook Pro:

Batteries - Maximizing Performance - Apple Support

About battery health management in Mac laptops - Apple Support 

Roy Bradshaw wrote:

Thanks for all the replies. What started me thinking was, my latest Non Apple Mobile phone states it ceases charging once the battery reaches 80% charge, I have also met this elsewhere, and wondered what Apple's approach was.

Read the information at the links provided by DI Johnson above. Basically the same approach. Neither the computer or the battery will be damaged by leaving the computer plugged in. The system will not overcharge and is managed very well by the MacOS.

Catalina software 10.15.5 and later for MacBook Pro with T2 chip (2018 models and later) includes a feature called Battery Health Management. Now, based on your usage patterns, this widens the hysteresis to initiate a charge cycle at a lower level, and stop well before 99 percent. 

About battery health management in Mac notebooks - Apple Support

About battery health management in Mac laptops - Apple Support

Battery Health Management feature tracks your usage history and detects that you do not generally run your battery down to low levels. So when you see less than 100 percent as a full charge, it is charging less than the highest level for longest battery lifetime.

NOTE that charging to 80 percent is a side-effect, NOT the GOAL.

The goal is to have some hurly-burly around using your battery, either by your actually using it, or by leaving it less than 100 percent charged, so that the cells are not 100 percent charged 100 percent of the time. Battery Health Management will also from time-to-time let the battery decline slightly to 'get some exercise' and run lower (thought to be around twice monthly) if you do not do so by your ordinary use.

The feature tracks you actual usage. If you continue to not require 100 percent charge for a few weeks, it generally will revert to a lower "full" level. but it takes some experience, which takes some time

This relaxes the set points around re-charging (based on your usage patterns) and can improve long term battery lifetimes.

If your recent usage patterns demand top battery capacity, the battery will charge completely to be sure you are not stuck out in the wild with no power.

Punchline: the Big Advantage or leaving it plugged in is that you need not spend a MOMENT of your time worrying about the battery charge levels. The computer does that computer-oriented job for you.

If working at my desk, I leave it plugged in.

And, contrary to others, I do not usually shut it down.

I just put it to sleep, whether for the night or when travelling inside my backpack, from home to work and back.

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I can't speak to keeping the laptop plugged in all the time but I'm old school in that I shut my Mac down at night. This clears out temporary system and application cache and swap files which helps in performance. Sometimes a cache file gets damaged/corrupted and can cause performance issues.

There are others who leave their Macs on all the time and just sleep them when not in attendance.

I turn my computer off if I won't be using it for an hour or so. Why leave it on? I also leave my computer plugged in whenever convenient to do so. I've never had issues during decades of computer use related to not leaving the computer on or leaving it plugged in.