The issue was too big of a dark dried fruit flavour (especially dark cherries) and an initial borderline acrid tartness that was unpleasant when mixed with the roasty stout elements. With age the edge from the sugar fades away but the beer was still desperately lacking balance and a bit too far on the sweet side (although that could be fixed with more bittering hops). More a weak, black dubbel with an odd vaguely roasty off taste than a stout with nice subtle fruity and spicy notes which is what I look for in a Belgian stout. Again, this could all be an issue unique to my candi sugar (it doesn't really fit the description in the linked thread, probably as a result of water chemistry), or I might have just plain botched that batch of syrup (although I recall it tasting fine before being added) but still. I find special b just that much easier to balance for both flavour and body in a stout even if it isn't the most authentic Belgian ingredient or whatever. Hell, both the special b and candi sugar might be redundant in the first place, as I find you can get nice distinctly Belgian results just relying on the yeast alone.
As a note on controlling fermentation temps: I find it isn't a matter of there being sugar or not, it's just with all Belgian yeasts period. For the first 24 hours keep it as close to 18C (64F) as possible (it can get a couple degrees hotter, no big deal) but after that initial day ramp up the heat; in this case probably aim for 24C (75F). Never had an issue with fusel alcohols or overly medicinal flavours with this approach.