As a miles card, what are some of the limitations of UOB Preferred Platinium Visa?

The very narrow definition of online spend makes the 4mpd online hard to hit.

If you can use it for online shopping and entertainment, then 4mpd (cap $1K) is probably the way to go.

Everything else about the card is just not appealing to me.

The $5 blocks, SMART points and expiry on the points just make this a card that requires close monitoring, and sometimes, having a extra card to maximize spending might not be worth the administrative hassle. So if you don't spend alot on the bonus category, then you might just want to forgo this card.