Completely depends on circumstance - the type of property and area is obviously going to affect the type of tenants you get, you're not going to get housing benefits/universal credit people in some expensive Central London or Canary Wharf flat as their benefits won't cover it. You'll get professionals and you'll do background checks or get a third party company to do so.
I rented for several years after first moving to London, maybe spoke to the landlord twice or three times a year during that time, obvs there was the annual gas inspection byond that it was just minor stuff.
The obvious thing to do from the landlord's pov was to have a handyman that you use for various things, both a landlord during student days and the landlord when first working in London had an odd job guy who would pop round and fix stuff if needed. Sometimes I'd fix basic stuff, things like a new shower head etc.. would ask for a new one, the landlord approved it and I'd just buy one and send him an invoice/knock the cost of it it off the rent for that month.
Likewise, if you buy a 3 bedroom terraced property in a student area in some northern city and you kit it out with cheap but robust carpets, convert the attic + one of the downstairs reception rooms to a bedroom and fit a single bed + computer desk in each bedroom then rent it out as a 5 bedroom property guess who your prospective tenants will be.
If you've got a particularly desirable student property then maybe you get their parents to act as guarantors, either way, you'll probably make them jointly liable, if one of them drops out then they need to find another and/or still ensure the monthly rent is covered and their mate or their mate's parents still pay them.
My wife and I both manage our buy to lets ourselves, my experiences with agencies have been poor to dire, their idea of vetting is not like our own.
Yeah, that's what I'd worry about, I reckon as an individual landlord you could probably put in more effort into vetting than an agency does, particularly things like references from previous landlord etc..
Realistically this is probably the biggest pro in having a good rental agent, keeping you onside of the legislation.
The thing is how do you know you've got a "good" one - it doesn't seem to be a regulated profession and it's letting agents rather than individual landlords who have managed to get in trouble over the "no DSS" thing in court.
https://commonslibrary.parliament.uk/research-briefings/sn07008/
I remember there was a sting by some journalists too where they posed as a landlord and told the letting agents they didn't want any black people, a bunch of letting agencies didn't bat an eyelid.
https://www.bbc.co.uk/news/uk-england-london-24372509
I suspect that it's not too hard to avoid tenants on benefits, if people are going to do blatantly silly things like individual landlords sticking "no DSS" in an advert or letting agents literally telling a tenant a property isn't for them because they're on UC/housing benefit then...
I guess if you're a landlord buying a very cheap property in a very cheap area and it's not a student area then you're going to have to anticipate that you likely will be dealing with people on benefits and act accordingly - in that case perhaps a managing agency is useful.
I think you're right to point out that time is a big factor in some circumstances, the yield might suck when it comes to more expensive properties but they're lower risk for that sort of stuff.
Some of the high yield slumlord-type properties (and in some cases perhaps student properties too) are probably best left to the full-time or semi-professional landlords, developers etc.. who live locally and have time to manage them. A couple of dozen cheapo properties in some cheap area of Manchester/Liverpool/[insert northern city] is probably a near full-time job in itself. In fact, anyone who has built up a serious BTL portfolio themselves and is managing it full-time probs has local handymen etc.. on call and is missing a trick if they don't then set themselves up as a managing agency for other landlords.
That's what one of my mates ended up doing up north, he started as a student and kinda decided getting a job wasn't for him, he's now the owner of a bunch of student properties and some other cheapo local ones and acts as a managing agent for a bunch more local landlords. Suits him as he's not in his mind doing any "work" per se rather he takes calls and then dispatches one of his guys to go fix stuff or calls one of the local electricians, plumbers etc..
He's done it for years now so doesn't take any nonsense - he once refused to give back most of a deposit after a house was left in a poor state, had a female law student write a lengthy e-mail to him about how they were going to appeal etc.. he sent back photos of the half dozen used condoms they'd found down the side of her bed when inspecting the property, told her he was happy to forward on to her parents or send as evidence if she fancied appealing etc.. she quickly backed down.