I too am a landlord though I acquired mine by marrying my partner-not sure what I could have done about that apart from marry someone else!
I've never made any money off the properties in nearly 20 years (they're all flats) and I try and keep all rents below market rate and don't increase rents for long-term tenants. But every year, service charges increase by at least 10% (insurance increases, utilities going up, increased staffing costs), and then there are repairs to the building—2 of my properties are in buildings that need new lifts this year at a charge of at least £1k per flat. I currently subsidise all my properties and have done so for years.
I don't pass on increases to my tenants as it's not their fault and I do run them as a business (despite them not being taxed as a business!) but it is bloody hard work and I'd love to sell them. They are all, however, worthless due to cladding and fire safety issues, which are taking years to sort out.
I've sold 2 to date—one for £1k more than I paid and the other at a loss to a long-term tenant, so I sold it to them at 10% below market value which seemed fair. It then turned out they owned 3 other properties in the building they rented out...
I wouldn't dream of chucking a £300 increase at a tenant on a rent of £1400 (I stick to 2-3% increases when I have to increase), but I do have a great long-term tenant who pays about 30% below market rate, and when HMRC audited my accounts, I got the Spanish inquisition over this as it seems it looks suspicious and they assumed I was taking cash as well. I spent 6 months arguing with them over this and other idiotic questions they came up with. So, charging below-market rents can cause you issues!
Personally, I have more of an issue with the profit banks make over mortgage interest rates and bloody HMRC's taxation rates for landlords, which lead to high rents, though there are definitely unscrupulous landlords out there. Just like there are unscrupulous people in most lines of business.