First of all, take a look at the encounter building equations from XGtE, which is much, much better than what's in the DMG. Since these assume a Medium encounter, add a level or three to the party for more difficult encounters.
Second, realize that all encounters are assumed to be beatable, with Deadly only providing "a chance" of a PC death. I've thrown out Deadly x3 encounters at a high tier 2 party, watching them obliterate it with ease. I've also had a Hard encounter at the same low tier 3 party kill 2 PCs (which were raised later) because their tactics and die rolls didn't go well.
Third, use 6-8 combat encounters per long rest, which is vital to the encounter design assumptions of 5E! I know this is unpopular, going against what the majority of groups enjoy. However, 5E combat was designed assuming a constant loss of resources over the adventuring day. If you're going to run fewer combats, you have to make them worth the full daily allotment of XP in the DMG, plus it's going to greatly increase the effectiveness of long rest classes.
Fourth, looking at XGtE again, see how many and what types of magic items you've given out. Many magic items are just more powerful than others within the same rarity. XGtE breaks the DMG magic items down into minor and major items, with minor items being far, far more common. Additionally, it assumes that from levels 1-20, that the entire party would collect a total of only 100 magic items. Most DMs love handing out magic items, and players love getting them, but since 5E is designed assuming the party has no magic items, each one you hand out is going to unbalance the equation.
Fifth, don't let your players pick their magic items. There are some very game breaking magic items, and I simply refuse to have them be found. A +x Shield is a huge boost to AC, especially when combined with +x Armor, making said PC almost unhitable. Gauntlets of Ogre Power can be really useful in a build that expects to receive it at early levels (allowing for a really low strength at creation). If the players don't know what to expect, you can give out carefully chosen items that won't unbalance the game.
Beyond that, it's going to severely depend on your group. Casual players are not as likely to need much balancing, since they won't generally find abusive/optimal combos based on feats or magic items. Serious players are going to see obvious stuff (the really good Feats, for example) and figure out better uses for magic items. Some players are going to min/max the hell out of everything, in which case you are going to have balancing problems even without feats and magic items! Good luck!