SQL CE 4 is a CTP and it is the only version that works with mojoPortal though future versions should also work, older versions will not.
Is it possible that SQL CE 4 has some bugs? yes
Is it possible that the mojoPortal data layer for SQL CE has some bugs? yes, it is the newest and least tested of our data layers. but if you find and report bugs they will be fixed quickly.
There are always risks, but each person has to decide for themselves what are acceptable risks for what they are trying to do.
SQL CE seems like a decent choice for a low to medium traffic web site but it cannot handle anywhere near as much traffic as using SQL Server. However the risk of outgrowing the capacity of SQL CE is somewhat mitigated because you can migrate to SQL Server fairly easily later if needed.
If I was putting up a mission critical bet my business on it web site, SQL CE would not be my choice. If I was putting up a web site for a hobby or a friend or other scenarios where I don't expect tons of traffic, the easy deployment of SQL CE becomes more attractive. Given that the vast majority of web sites on the net get very little traffic there are probably numerous situations where it might be attractive but it is a judgement call for each situation. For a guy like me who is very experienced with SQL Server and has no trouble setting up new databases and automating backups the attraction to use SQL CE may be less than for someone who is not skilled with SQL Server and needs it to be easier and to just work with no fuss.
Hope it helps,