I have been a very small time investor on Sharesies for nearly a year now, and there is just one issue with this platform that bugs me: share ownership is through Sharesies Nominee Ltd. So with everything purchased through Sharesies, the registered owner is the nominee company - not you. If you buy shares in a company it is the nominee company that is on their share register. As a consequence you do not receive shareholder communications (such as reports, AGM invitations etc) and you cannot exercise shareholder rights (such as meeting attendance and voting).
For the most part this does not bother me as I am a very small investor ($700 in total invested through my Sharesies wallet) but I transferred in an existing shareholding (Vector) worth over $3000 without realising this. I am no longer on the Vector share register because of this.
So Sharesies is not for investors that wish to actively participate as a shareholder in the companies they buy into. It is, in all other respects, a good platform for entry-level participation on the stock exchanges they offer (principally the NZX).