What are the downsides of using surfer compared to say the same server without cloudron using nginx or apache
My instinct is to rephrase : what are the UPSIDES of using Surfer ....
But to answer the question, downside of Surfer is it does one thing serve files. Having a server with full range of facilities opens up possibilities (which you may not actually need/want) but significantly also carries management and maintenance. If 'set and forget' is what you prefer, then Cloudron and Surfer/LAMP is much better than having a traditional self-managed server.
And if the question was LAMP on Cloudron vs traditional server, then again the LAMP will almost all of what you would normally do on a normal server without the maintenance headache.