Pros -
They'll be shit hot on things like elec testing, fire recctification works etc. Probs got a basic redecoration schedule etc. They'll manage all communal facilities (repairs, lighting, cleaning etc).
You'll be a leaseholder, so will pay the HA a monthly contribution towards communal services and a sinking fund for future major works. These are regulated, with caps on how much they can pass onto you, and at what rate.
You'll have a willing buyer if and when you come to sell, as they will likely buy it back from you (albeit not at market value) if when you come to sell, you are struggling.
You'll be able to liaise with the HA about noise complaints, asb etc, without having to deal with a remote agency who manage the lease.
It's possible you will be able to book repairs and access their range of repair staff, at a cost, for works inside your property.
Cons:-
Mix of tenures means mix of people and everyone living on top of one another can be quite suffocating / challenging when different people behave differently.
Less attractive. Unless you are really into that sort of communist cement genre, they aren't all that pretty (though I absoloutely love the look of a lot of HA builds).
Monthly service charges would frustrate me as a home owner as they do fluctuate annually.