For me, the only really awful novel he wrote was 'Mark of the Beast'.
'Friday' was pretty cool, imho.
My daughter is reading 'Stranger in a Strange Land' right now and is enjoying it, as I did. It was a huge hit and has been in continuous publication. I think someone is trying to find a movie in it.
'To Sail Beyond the Sunset' was not awful but it was tedious, even though it redeemed some of the screw-ups in 'Mark of the Beast'.
If I recall, he supposedly had an undiagnosed blood flow blockage to his brain, with 'Mark of the Beast' being the most heavily impacted story. I think he wrote very little after "Mark" until he had a surgery to repair the problem.
But I do agree that his earlier stuff was his most exciting and well written.
Ed