I have had now 4 of my properties go onto the HAP, two with existing tenants in place. I have to admit that I was shocked my two existing tenants were able to claim this as they both had reasonable jobs, but its not my job to query this.
The only negative is the HAP pay in areas where as tenants usually pay in advance, which will seem like you are missing a months rent, but you will get an extra one when they move out.
If you have filled out the paperwork once it's really not a big deal. The tax clearance cert is easy to print out from ROS, send them a screen shot of your online LPT local property tax page to show you own the property and a bank statement showing the IBAN of the selected bank for the standing order has the landlords address on it. You can then claim 100% of the mortgage interest instead of 80%, although you have to wait three years before you can claim it all back.
What's important here is that it shouldn't matter how you get paid by your tenant. The personality, honesty and integrity of you tenant has not changed. They will hopefully be just as likely to leave your property in a respectable condition regardless of how their rent is being paid.
But both my existing tenants asked me if going on to the HAP was possible. They certainly did not go on the assumption that I had no choice. To be honest they absolutely LOVE ME NOW