Though Labour tries to portray Jeremy Corbyn as a strong leader, under his tenure, it hasn't successfully or cleanly dealt with many of its own internal issues. There's been a slowly bubbling scandal around antisemitism for quite some time, and infrequent-but-persistent infighting, with Momentum often mentioned as causative culprits. The effects of this internal disorganisation and lack of discipline are plain to see on a day-to-day level, and there have been many divergences between what Jeremy Corbyn says is Labour policy, and what his shadow ministers will say. If I'm not convinced that he can competently run his party and heal it's internal division, then it's hard to be convinced that he can competently win an election campaign and go on to run the country, and heal it in the post-Brexit period.
It should be noted that the same issues exist within the Conservative party too; it has a problem with islamophobia (maybe xenophobia and jingoism too), and the ERG is a divisive party-within-a-party just like Momentum is.
Speaking of Brexit, Labour has dithered and dathered on what their policy actually is. If there's one thing that's obvious about Brexit, it's that many people and businesses in the UK (and outside of it) would massively benefit from certainty... yet Labour has flip-flopped between various stances, none of which have been definitive and easy to grasp.
While I appreciate that the debate is nuanced (something the Tories missed when they decided to run a yes/no referendum), Labour's indecisiveness to settle on a position has meant that they haven't provided a cohesive alternative to the Tory government, which has contributed to them failing to provide an effective opposition which can hold the government to account. I mentioned this above too, but it's been especially apparent and important throughout the Brexit crisis.
Their current policy is to have a referendum on any deal that the UK Government does negotiate. I like the idea of a referendum, but only because I hope that it would stop Brexit altogether. I think the process of yet another referendum (presumably after the election?) would be hugely tiring and divisive.
As of a few days ago, the Labour party was still actively taking submissions for what should go in it's 2019 manifesto, which isn't exactly encouraging considering that it's already November, and the election takes place in about a month.
Apart from that though, their website has lots of policies that sound good; Labour wants to strike a Green New Deal (essentially upgrading the country to run sustainably, bringing lots of investment and jobs), increase the National Living Wage to £10 an hour and ban fracking. It's also worth noting that a Labour Government would also put more money into basically all public facing government services. Of course, this includes the police service like the Tories want to fund, and their website implies that they'll invest a similar amount into it too (though Jeremy doesn't bluster about that anywhere near as much as Boris does).