So 5th edition as it stands is a trickle of content from the main developers, but the best part about this trickle is that has encouraged so many homebrew and 3rd parties to get into the thick of things. The only problem I have seen from it is the purist that only want to use official content but still complain “There are not enough options” and usually talk about how pathfinder and 3.5 have all of these options. But the silly thing is, a lot of those options have been ported in one form or another by 3rd parties or homebrewers.
Some of the content 3rd party game developers have been great work. Sometimes it outshines the content of Wizards of the Coast dare I say it. But at the end of the day, it is an option that a dungeon master can either agree or disagree on adding it. Which can be pretty daunting for the Dungeon Master, because they do not want to enable something that is overpowered and might impact the rest of the group, so they feel uncomfortable with letting it happen. They do not want to ruin the fun for others and might be hesitant on letting 3rd party work at their table. The thing that always confuses me is if your players are not going by official rules, then why should you? You can come up with plenty of things to try to give balance for the rest of the players later on that will give everyone an even playing field. After you do that you can always up the encounter difficulties and give your creatures more strategy. Not every creature is just a target dummy, they use tactics and are very aware of their surroundings. These are all things that will increase the difficulty. If players are still coming out on top without a sweat, you might want to look at how many times you are giving out long and short rests and figuring out of ways of limiting those. Players will not always have access to resting spots and limiting those is a good idea.
If each encounter is going too well, maybe up the anti… reinforcements arrive and they are here to rumble. Maybe they get a surprise round because your players were not aware of them and they got into position. Battles can be hectic and unless your players are saying they are actively looking out for extra creatures joining the fray, they will start to after you do this a couple of times. Making it so they might have one character skip a few turns keeping watch often. What I am saying in the end, is don’t be too rigid as a player or Dungeon Master, 5th edition is a game of creativity so get creative! Use someone else’s ideas to inspire you or take them! Because at the end of the day, it is about the fun you and your players have that matters.