Can amelia -spd +rest be strong?

Absolutely. I'm gonna preface this by reminding you that Amelia is an incredible unit, and it going to be buildable and a good unit no matter her boon/bane, just like my +speed -def Hector is still a great unit. In fact, much like my Hector, Res is the one stat that Amelia gains four points in if it's a boon. This means a few things. Instead of a thin 23 res, she has a fairly beefy 27 res, complemented by her large HP and good speed. Even with a speed bane, she still outruns hector and the like by far. The only armored unit who comes close to her speed is Draug. Also, because she gains 4 points in Res, added to her having the highest base total of all units released to date, your Amelia will have the highest possible base stat total, which means higher arena scores. You could build her to be a tank like Sheena, who can take hits from both physical and magical units, and if you have distant counter to drop on her then she will of course appreciate it. Otherwise, her regular earth boost is pretty darn good, even better if you have a +hp seal. Or distant defense could work for more anti-mage bulk. Then Quick Riposte to charge up her special even faster than it does with Slaying Axe+, and a special like ignis or bonfire. Alternately, you could run her in Armor Emblem, with hone armor and goad armor, making her reach 41 speed even with her speed bane, which is quite ridiculous, and allows her to double a huge amount of units. More with the speed seal. If you do run her under these buffs, you will probably get more milage out vantage or desperation, since she'll be doubling units naturally without QR at that point. Basically, your Amelia might not have a ridiculous 34 speed like you'd hoped, but still a fast 31 for armored units, and it means that she now has no weaknesses. I would be very happy to have an Amelia of any sort.