Assault creates boring, drawn-out games where even the smallest mistake by either side heavily swings the match in the others favor. This is not an interesting game dynamic and at the very least assault needs to be tweaked or, in my opinion, removed entirely from Team Arena. This is just my anecdotal opinion based on my games so far so lets grab some data from 343i's API to see if this really is the case.
I gathered a random sample of 1236 CTF and Assault games (472 assault, 764 ctf) and analyzed their duration and victory conditions.
At 12 minutes the standard game time runs out. If at 12 minutes one team has more points than the other then the game ends and the team with more points is the victor. If there is a tie when the timer runs then overtime begins and the same check is done at the end of overtime. From the chart above it's quite clear that Assault games are much more likely than CTF to either end due to the timer running out or in overtime. This is indicative of low-scoring games where neither team is able to reach the required 3 points to win.
The red group in this chart indicates the percentage of games that ended due to a 3-cap (a team managing to score 3 points) before the initial timer runs out. The blue region shows the percentage of games that ended from a time-out (one team being ahead at 12 minutes) or a victory in overtime (one team being ahead after overtime or reaching 3 points in overtime). So nearly 25% more Assault games than CTF games end from the timer or in overtime.
I don't have a problem intrinsically with the fact that Assault often produces long games, if the objective was an interesting one. The singular objective in Assault of 1 ball and 1 cap location produces games where decision making means less than in other game types. Since there is no question about what you should be doing at any one time it takes away decisions from the player.
My other main issue is the variability. Under normal gameplay it's very difficult to cap the ball in Assault, as I have shown here with their abnormally long games. However, a single lucky kill (or unlucky death) can easily swing the game in the your favor (or your opponent favor). It's a mechanic that isn't satisfying because it feels like you're playing for that lucky kill instead of making smart decisions throughout the match.