I have 5-lug and wouldn't go back but for specific reasons. First your questions. There are only three disadvantages that come to mind.
1. Initial cost, including a collapsible spare tire and electric pump. Both are readily available.
2. E-brake likely goes away and I occasionally miss it.
There are work-arounds, both hydraulic and mechanical. That said, I have 10:1 compression that works fine in most cases. But I can't trust it on steep grades though.
3. Bigger tires generally cost more money. But there are way more options
My car was half converted when I bought it since it already had 911 Bilstien front struts and 5-lug adapters in the rear. My excellent PO had the 914-6 rear stub axles though, he had simply not done the conversion yet and they came with the car as did an extra engine and lots of other wonderful things.
All I needed was the rear wheel hubs and I was in business. PCA and SCCA wouldn't allow adapters so I just completed the conversion because I need the wider rubber. No brainer.
I have converted to 911 stub axles and 944 CV's so I still have my 914-6 stub axles if you're interested. Haven't thought about selling them but I no longer need them.
Good Luck!