A great website is of no use if with a rubbish web hosting provider and there are a lot out there so do your research. Of course no hosting provider can guarantee you 100% uptime as this is not possible – this does not exist.
A fact of operating website servers is that from time they go down or need services. Most providers do this off-peak and try and keep it to a minimum but it does happen. What you need to know is that a web hosting provider will have some downtime. Even if this is just a few minutes a few times per year it should be of no issue.
Security should be a concern though and less robust hosting services can be prone to security breaches. It is best to do careful research on th. Most hosting companies provide shared hosting where your site is on with many others (this keeps costs down and makes sense in terms of space).
However if one of the other sites does get hacked you may be prone to getting hacked as well so keep regular backups and update your site software regularly.