Even in the 3.5e times Sunless was a bit of a slog for 1st level characters, with too many giant rats and goblins to face head-on to not be killed or forced to retreat. Not to mention you can run into a white dragon wyrmling? young white dragon? whose breath weapon can easily kill full HP adventurera. 1st level resources are so small that any adventure at that point should be like 3 encounters with a short rest in between each-Starting at 3rd level will be a better bet.
That said, Sunless Citadel is still one of my favorite adventures. Very good mix of dungeon delving and small-scale adventure, and you can use Belak and the Gulthias Tree to kick off any number of adventures. I was even considering having the tree drag the adventurers into Ravenloft next time I run it.