I disagree that Tuatha are weak, thus don't agree that the bards should receive a research bonus. Sure, they are fairly weak as far as research goes, albeit actually :
22 themes have noticeably better research.
7 themes are approximately equal.
8 themes have noticeably worse research, 6 significantly so, with their best researchers costing at least .5 gold more per Research Point, 1 whose best costs over 1 gold more per RP.
(It's hard to get a really good comparison, but since many nations take at least Magic 1 if not forced to take drain, I compared all with Magic 1 for 30 turns.)
Of those that are roughly equal or worse, some are generally considered "weaker" than Last of the Tuatha : Machaka, base Ulm, base Ermor, and if you believe Cohen, Abysia.
So - LotT are slightly down in the bottom half of the barrel research wise.
Balancing that, they have one of the 3 best national commanders in the game, great for spell casting, great for combat, and having recuperation so their wounds go away - you can even put disease inflicting items on them. True, they're capital only, but they can Air Trapeze without needing any booster items.
Actually, I think the only weakness of the Tuatha theme is that basically
everything good is capital only : their two best commanders, and their two sacred troops. But again - balanced by having
great sacred troops; not quite as wonderful as Vanheim's but significantly cheaper.
And one of their capital only commanders is actually a cheaper researcher than the bard, if you go 50 turns or more, because it's sacred.
Like many nations, you simply want to find independents to do the heavy lifting on research.
The thing I think LotT could use for improvement is having at least 1 of its sacred troops or sacred commanders not be capital only.
And also, Man is one of the very few nations that gets
worse PD for each point over 20 than it gets for points under 20, which seems flawed to me. (Man and Pangaea each get 2 units for PD under 20; over PD, they simply lose one of the units, without getting a better unit type.)