Essentials does not modify skills and/or items added by SynergiesMOD. That means the endgame gear is still the best. It does give you better legendaries throughout the game but that's not a bad thing as they are pretty bad in the base game.

There are only a few things I could think of that could break the balance but all of them are completely optional:

#1: Legendary Pet transformations. I have never used them since the pet is not supposed to be very strong if you don't have the required skills/gear. I would stick to the regular starting pets and your class summons.

#2: The socket vendor in Mapworks lets you add 5 sockets to any item but the default is "up to 2 total". The only exception I allow myself is for 2h weapons to have 4 since you're missing the off-hand sockets.

#3: The enchanter vendors in Mapworks lets you enchant up to 5 times when the base game limits you to 4. Once again, just stop at 4 and you'll be legit.

#4: The socketables from Reiners mod (actually, all items from reiners are unbalanced but they're still weaker than T0 at the very least). These includes de "moon stones" and "dragon scale" This is the one thing I would avoid at all cost.

Aside from that, it's mostly UI and QoL improvements. The rest of the new mechanics are not that great so I wouldn't worry about their balance.