No, you are not unbeatable, the are many ways it doesn't work
Not counting the odds of hitting every time for this to work, I list the major ways to nullify this combo.
1. There are range characters, like archers and spellcasters, who don't care about melee feats, and can kill you from hidden long distance (bows 400-600 ft.) or be flying out of reach and spell snipe (120-600 ft.).
2. Many martial characters have access to Misty Step from multiple sources and can "Bamph" beside you as a bonus action, then attack you. Metamagic can Quicken any Teleport Spell
3. Have the Mobile feat and throw a dagger/javelin from at least 15 feet away, then finish moving up and attack, now that you have nullified the PA threat (Swashbucklers can do this too with the 3rd level Fancy Footwork feature).
Mobile Feat 3rd Feature:
When you make a melee attack against a creature, you don't provoke
opportunity attacks from that creature for the rest of the turn,
whether you hit or not.
4. You can obtain the ability to use Disengage as a Bonus Action through the 2nd level Rogue feature. Then Declare as a BA, run up for full attack! And if he steps back, you get an OA, and can do this all again next round!
If you take the Disengage action, your movement doesn't provoke
opportunity attacks for the rest of the turn.
5. Everyone can just declare the Disengage Action and run up and stand beside you. After doing that, you can only use a Bonus Action (cast a Bonus Action Spell, like Hex/Hunter's Mark, or a Quickened spell, or use a bonus action attack/effect) until next round, when you can go full attack on them. They can step back 10 ft., but this gives you an OA.
"But wait! Sentinel allows me to hit creatures who are using the Disengage action, so I still get the OA!"
Unfortunately, Sentinel doesn't work that way, it is very specific about when this special ability applies.
Creatures provoke opportunity attacks from you even if they take the
Disengage action before leaving your reach.
Since they are entering your reach, Sentinel does NOT apply.
So thus "unbeatable combo" is quite beatable, and in some cases a bad trap for the person who thinks this "unbeatable".