There are no health downsides to a longer veg period, as long as water and nutrition are adequate. The only risk is running out of room for the roots and the physical size of the plant. Remember too, to match the photo period (day length) that your plant is receiving to the outdoor day length. If you have your plant indoors at 18 hours then place it outdoors too early and when there is only/less than 12 hours between sun rise and sun set, your plant may start flowering. You should really wait until after the equinox before placing the plant outside, unless you add artificial light to supplement the day length to 13 hours or so. You should also have your plant at the moment at 13 or so hours of light so that it matches the outdoor day length once you put your plant outside.
Hope this helps,........
Organoman.