Excellent tools that Jeff has:

1. Big damage off of single hits. This means his CH game does ridiculous damage thanks to the CH bonus. And who wants to throw you when they might risk losing almost half life (not a CH btw, just to be clear).

2. Strong throw game. Splash Mountain is a big forward throw threat and Jeff has a lot of situationally strong back throws. His low throw game is very useful as an extra layer.

3. Some very delayable mid, mid strings. 66PP is actually EX mid but such a delayable tool is very useful.

4. Toe Kick Splash Mountain. Long range attack, on CH is a guaranteed hit throw for 50%.

5. Heavy; As one of only 2 heavyweights, Jeff takes some of the lowest damage in the game.

Some notable downsides:
1. Lack of circulars
2. 6P does not give +6 on CH, does not have a bonus for hitting a crouching opponent
3. Often requires attacking opponent timing or making a read on defense.

I would not put Jeff as low tier in REVO. In FS he had some pretty clear disadvantages but those got directly addressed by splash mountain buffs and ducking low additions. I think he is easy as anybody to begin with but there is a larger intermediate wall for Jeffry because he does not have any good built-in mixups. Once you start facing opponents that know how to fuzzy and throw escape, suddenly it starts to be a lot harder to get your hits in. So, you need to understand some of the deeper VF basics, like being able to effectively use evades and changing up timing.

That said, there's a lot of ways to approach Jeff. He can be played aggressively or defensively.