Sweetspot, AKA high tempo/low threshold, is a great way to get aerobically fit during the base period. You can do two low-end sweetspot sessions during the week to complement your strength training in the gym. If you enjoy interval structure or are riding indoors on the trainer, add one higher-end interval set at the weekend. Sweetspot saves time; it’s more efficient than sitting on the turbo trainer or indoor bike for hour after hour at endurance pace.
There is nothing wrong with doing sweetspot intervals during the race season, especially if you are short on time. Again, you should not rely on sweetspot alone. Although you might prefer riding at sweetspot to 100 per cent FTP sessions, remember that it is not a replacement for riding at 100-105 per cent of FTP, and you still need to do some over-under type intervals too.
Sweetspot may be “comfortably hard” but don’t get too comfortable and familiar with it – it is not a complete training plan in its own right. Once you can nail sweetspot intervals and increase their duration, you need to go harder so you don’t stagnate or plateau.