The biggest downsides are the pay and internal career progression.
Whilst the company has a solid foundation and good growth prospects, it only has a few major clients who provide most of its income. As a result, if there's been a setback for the company as a whole then it's not uncommon for salary increases to be delayed or deferred which in turn means that your salary can end up rising lower than appropriate for your level of experience - the salaries aren't bad as such, it's just that sometimes you could end up getting paid more for doing the same job elsewhere.
And because of the small size of the team it's also the case that room for internal career progression is somewhat limited. You can go from junior developer to a regular developer of varying degrees of seniority and that's about it unless you're able to make the jump to architect.
