Even with Impish and 252 HP/ Def, Chesnaught has a chance of being 2HKOed by a +2 Breloom Mach Punch after rocks, whilst being incapable of OHKOing with anything other than Aerial Ace. Adamant Chesnaught with Poison Jab is the only competitively feasible Chesnaught that can take on Breloom, being the only variant capable of OHKOing, and it's faced with an unpleasant choice:
a) Forgo Synthesis, meaning that after one mistimed switch into Mach Punch, Chesnaught is in KO range of a single +2 Mach Punch.
b) Forgo Bulk Up or Swords Dance, and get cockblocked by Skarmory/ Forretress/ Scizor/ Gliscor/ Tentacruel/ take your pick.
c) Forgo fighting STAB and get cockblocked by anything steel.
d) Forgo grass STAB and get cockblocked by Crobat, Gliscor, Tentacruel, etc.
Chesnaught can't afford to set itself up to deal with Breloom. It requires too specialised of a set, which frankly would be useless at everything else.