I've had slightly different experiences playing T20s against AI (playing on PS4 with the latest patch against Legend difficulty).
On the whole, the AI batting seems fine. Fixing the running between wickets bug in the latest patch makes the AI keep the score ticking and reach competitive scores. The biggest problem with the AI batting still is (and has always been as far as I can remember) it's inability to take the end-of-match situation into account -- if 4 runs are needed of the last ball, the AI batsman would defend or play a tame ground shot. This completely ruins any believability, and worse, makes what could be a tense ending to a match a real anti-climax. I can't figure out why this is so hard to fix - even having special logic kick in for the last over of a match would make the gameplay more realistic.
The other big problems with the AI in T20s are:
1. AI bowling changes are scripted and inane - I would whack a bowler for 30 runs in two overs, and the AI would still bring him back for the third over, when there are other bowlers who have bowled better in the match
2. AI field placements are ridiculous at times - e.g. the AI would you start the last over of a T20 match with an aggressive field (eg. with 6 infielders), and worse, have fielders up in zones where I've already hit down the ground previously (e.g. have mid-on and mid-off up when I've already hit several 4s/6s straight down the ground earlier in the match)
If these few issues can be fixed, I believe it should be the best limited overs experience in a cricket game.