Falinks is an interesting pokémon. When I first saw it, I was like, what type could that be? Bug? Kirby? And to my surprise it was Fighting, with decent Physical attack and defense as opposed to its other stats. But now digging deeper, even it's great ATK and DEF fall by the wayside. Base 100 DEF is nice but Base 65 HP means you are only moderately bulky. Base 100 ATK is scraping the barest realms of usability outside of abilities for generations now and Base 75 SPE along with questionable defenses (65/100/60) means Falinks is hard pressed to attack or defend.
That is where No Retreat comes in. As Z-Conversion and Z-Celebrate have proven in the previous generation, a +1 to all stats is rather significant, enabling the user to become significantly more bulky on top of gaining increases in Power and Speed. However, this comes at a cost -the ability to switch. While this may not seem so bad, if you pop a No Retreat too early, Falinks may become setup fodder, as No Retreat is a one time use (unless the user is forced out) and when running No Retreat, Falinks suffers heavily from 4-MSS, unable to hit everything it wants to.
Jolly Falinks with Max Investment is frighteningly fast after a No Retreat, outspeeding Positive nature Base 130s like Jolteon (409 vs 394), but will be outsped by anything faster with a Scarf/Speed Boost (Galarian Mr. Mime, Haunter, and Rotom-Frost come to mind as likely candidates).
Close Combat being Falink's STAB of choice is a double edged sword. While the power is incredible, it ruins the DEF buff given by No Retreat and makes Falinks that much more vulnerable to being KOd by priority. Additionally there is the matter of finding a moment to use No Retreat, as Falinks is not especially fast and is prone to being dented/KOd or taunted before even getting the No Retreat up.
However, this is what makes Falinks great for late game where its checks and counters are gone, so it can come in, setup and sweep.