Firesweep Lance Revisited

It really depends on the unit. Never use firesweep on a unit that can't double, because then it is completely inferior to Brave Lance. A boon of firesweep is that unlike Desperation, you don't have to take damage first. You can start the battle kicking and be able to take a single hit if you have to. On movement type specific units (Cordelia, Roderick) it is definitely good cause 99% of the time they'll be able to take out a unit in one round with buffs (Other than Axes). Another thing that is good about firesweep is the ability to do chip damage on a bulky axes. With hit & run + a unit with reposition/draw back, Hectors and Amelias easily fall to the firesweep Lance due to the lack of movement. And then there's also the fact that it counters vantage and allows you to avoid the other units specials. They could have an Ignis or something charged up and end up one shotting your unit on the counter attack without Firesweep, but with Firesweep you avoid it completely. Firesweep is good, not on just any unit, but it is definitely good.

Well It depens from unit to unit.
True making for 4 Attack on a raw is Pretty scary and powerful especially It then add skill like galeforce.
Fs lance is good for few reason many unit cannot quad since the -5speed won't allow It and you Need to be a flier or a horse because their buff to do so.

Look example Cordelia with +Speed and brave lance She keeps at 33(unskilled and unbuffed) Speed which is fairly low and since she is a brave lance user She would prefer db3 to have more Power and If It happens to not kill on that round (which is rarely ) She Can still potentially take 1 hit. True with desperation She would kill almost everyone but She would use her b skills.on the other hand +Speed Cordelia would run Better fs lance since she is already at 38 Speed and since she won't get countered She Can run lnd Pretty well making her at 43 without buff good enough to doubled already. Having more raw Power than brave weapon She Can ensure the kill too without the risk or being killed (which quad desp would do too) however you have now your b slot free to use in different way. However +attack is a Better brave user since db3 +buff make sure to kill without quad.

It depens on situations example effie cannot run fs well because of wary fighter while brave lance is her best weapon.

As for infantry unit asside blucina and maybe srobin there aren't many fast lance user to use fs,(blucina would keep her weapon thou).

As for srobin She Can run fairy well since she has good offense (while her attack is not the highest so Better might help her) and good defences (asside hp) so She needs to be +speed/-res eventually She had doubled but hadn't killed someone She Can still survive due her defences however the Attack might be It She not get doubled ( but She established before that She had doubled instead), She would survive since many do not use brash assoult and the only special that She might suffer is moonbow but could potentially survive It you give her Fury(and If you do not consider enemy with killer weapon and -1 cooldown Seal and skill like Wrath).

A unit like Charlotte She has a very High Attack, however her Speed is not bad nor good too so even If She is +Speed She won't quad much and If She has fs She really Need lnd to ensure the double (which might actually good due her defences) but She would still prefer +Attack and kill in her First hits with brave lance since her Speed is mediocre.

In the end brave unit prefers Attack over Speed since they potentially Can kill during the First hits however if It happens to have a good Speed too you Might can still work for quad especially on emblem.

Fs unit prefer Speed over Attack since the Better might does the job and they Need to double in their own,Can run lnd and avoid vantage se well they have the b slot free.

As for tanking both of those kind are bad because:
Brave unit due their less Speed
Fs unit since they cannot Tank at all
However unless re has lnd, since It has higher Speed is less likely to be killed since they do not get doubled usually. Still It really depends on the unit and which gameplay you want to use them.

Summary
Effie:brave unit due wary fighter
Cordelia: both (+attack brave unit,+Speed fs )
Hinoka:brave unit due lower Speed compare to Cordelia despite having access to flier.buffs ( you would maybe Wish Cordelia +Speed and vs and +Attack brave hinoka 2 lance flier with different roles)
Scorrin:fs due higher Speed over Attack and good defences
Charlotte:brave due High Attack (fs only in +Speed +lnd)
Donnel:brave unit good Attack but really to alle
Blucina: could fs bnn her original weapon makes Better
Camus:neither,is weapon is good for him
Oscar:fs due his Speed and horse emblem
Other blu cavalier are not that relevant or i do not know to Say so