The general relation of unique girls' base stats from weakest to strongest, assuming level and presence of + being equal:
"Story" (the unique girls gotten ONLY through silver) (term comes from how we get free copies of a number of them through story missions)
"Common" (the unique girls available in BOTH silver and platinum) (my term due to them being the 'common' result from platinum recruit)
Event
Platinum exclusive/limited
Exclusive (the actually time limited exclusive girls who do NOT end up in Platinum)

As stated, that assumes everybody either has a +, or nobody has a +, to keep things equal. A + typically adds somewhere in the range of ~300 to HP/Attack/Defense at lv 80. It's enough to pull event girls+ up to just a bit ahead of platinum non-plus. You can see an example with Eins; xmas Eins+ is in between Eins (non-plus) and Eins+.

And in case anybody's curious, Liza fits in 'Common' tier, as her stats are incredibly similar to Felicity's, actually. Shizel's stats are plat-limited tier.

How worthwhile it is to get Bertha, or any event girl, to UR+ hinges on whether you're lucky enough and/or you throw enough money at the game to fill up your party with UR+ plat-limited or Exclusive girls. I assume that most people don't fit that description.

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Oh, and before anybody brings up HW Lilium+, think of her as like Lavender/Edelweiss/Mountain Lily in FKG. The event girl concept starts off strong, then the developers change their minds, and then every event girl afterwards is weaker than the oldest one(s). Except here, it only took 1 girl (HW Lilium) to change the devs' minds as opposed to like 3 girls.