The only downside I see is the stunted spell progression as compared to a pure Druid.
Admittedly I've never tried this build, so this is all theory. I also don't know how this would do at epic levels, but it seems feasible at levels 12-14, which is where you'll get during SoU.
1) spellcasting progression and caster level
2) shapeshifting - you will have to wait longer to (or may never) get access to more powerful forms.
3) animal companion - the strength of your companion is based on your class level as a druid.
The loss of spellcasting ability is much bigger than most people think. Don't forget that it's both your spell progression and your caster level. Your spells naturally grow more powerful as you level up, in addition to gaining access to more powerful kinds of spells. So for instance, an 8th level Druid deals 8d6 damage with flame strike, while a 10th level Druid deals 10d6 (a 25% damage increase!). The 10th level druid can also cast this spell more frequently due to additional spell slots, and has access to even more powerful spells that the 8th level druid can't even cast. As a rule of thumb, spellcasting abilities roughly double in power every two class levels.
You are correct in pointing out the level 12-14 range. The Druid is devoid of significant class features from level 13-15, so multi-classing in this interval is relatively less painful than usual. However, even at this ideal threshold it's still a steep tradeoff to give up spellcasting progression. If you don't cherry-pick an ideal threshold, a multi-class druid is just way behind a single-class one.
I forgot to take caster level, shifting level and companion level into account. The Druid/Thief would be behind the pure Druid in overall firepower, but I don't see this as a deal breaker. You said before, and I agree, that a pure Druid is one of the strongest classes in game, if not the strongest. I'm just about finished with SoU on Normal with my pure Druid and it was a cake walk, I only died once back in Hilltop due to my own stupidity. (I've got plenty of that.) So tossing in 3 levels of Thief will leave you at Druid 11/ Thief 3 at endgame with maxed out Call Lightning (10d6), able to cast greater stoneskin plus 2d6 backstab, evasion and uncanny dodge from the Thief levels. Sure it's not as good in a min/max sense as a pure Druid but it should be able to finish SoU plus you get the satisfaction of playing an unorthodox Druid build!
The OP was asking for class recs that could tank, sneak and heal. Maybe he could do a pure Druid and focus on the stealthy shifter forms?