Chelsea had a decent beginning, leading by two goals, but a lack of experience in certain positions, such as Robert Sanchez's, allowed Arsenal to level the score. I believe Chelsea could have won this match if they played with more experience. They handed the game to Arsenal when Pochettino reinforced the defense while they were only ahead by one goal, which seemed quite unreasonable.
In terms of inconsistency, I think Manchester United faces more issues than Chelsea, and the improvements they need to make are even more extensive than what Chelsea requires at this moment.
Chelsea not been able to defend their goal at home to secure a win was really disheartening, pochettino mad a mistake when he removed plamer and other in the later part of the second half, he changed to a defensive tactics maybe to protect the two goal he had already but it was wise enough to weaken the front line and strengthen the defence, as Arsenal to advantage of the less pressure on the attack.
It's was after these substitutions arsenal saw some space to penetrate well enough to get goals, Arsenal was able to exact good amount of pressure because Chelsea had a lesser pressure coming from their attackers to Arsenal, that have Arsenal the chance to send the pressure home at them, playing a team as Arsenal who is in Good for, weaking your attack shouldn't have been done at any point, instead he could possibly strengthen the attack and the defense by taking away some midfielders because if his defense is still solid the attackers can take advantage of a counter if it happens, that way he could get a balance.