This is quickly becoming my favourite secondary arcane. Giving up Secondary Encumber (and Surge in a few cases) was hard, but it's fairly easy to compensate nowadays.
So what's so great about Fortifier? It performs two roles: it gives overguard to frames that can't generate it but enjoy useful synergies with their native defenses, and it also rapidly strips overguard from affected enemies with a x8 damage to overguard multiplier, opening them to CC. CC might have died for a bit, but Fortifier has kind of brought it back.
I'll cover two points in this post: frame synergies, and my favourite Fortifier weapons. Let's start with the synergies.
Frame Synergies
Stealth frames benefit hugely from Fortifier for two reasons. The first is that they typically enjoy running Molt Efficiency, and the extra layer of overgating ensures Molt Efficiency is always running at max power. The second is that overguard broadly cancels almost all negative effects on the frame, including Conduit debuffs in disruption, Netracell debuffs, Archimedea debuffs, Eximus effects etc. One of my banes on Ivara was getting hit with Arson waves while I was on my dashwire and not able to react, or the punishing energy drain conduits working against Prowl. No longer, I am free! The same can be said for Loki for Disruptions and if choosing to run Irradiating Disarm or using Decoy.
Revenant benefits from Overguard for similar reasons, as some negative effects still apply to him even through Mesmer Skin.
Zephyr, my favourite frame of all time and the core of my Warframe addiction, became very clunky to play after Zariman and Overguard, but Fortifier, especially on a good rad weapon, means Turbulence can do its job and keep her very safe again.
Finally, and credit to the wiki for this, damage redirect frames, notably Yareli and Nezha, and to a lesser extent Nekros and Trinity, hugely extend the effective overguard provided by Fortify, up to 150k for Yareli, Nezha and Nekros, and 60k for Trinity.
Favourite weapons
Grimoire goes without saying. The pulsing AoE and chaining property means this rapidly strips overguard in a wide area. Tome mods provide great utility. Without Surge, I only use the Fortifier config on frames that can compensate the damage, like Zephyr and Yareli.
Akarius Prime is a nice hybrid pistol with a massive radius, easily spreads radiation and other statuses while shredding overguard.
Hystrix Prime remains my staple for Disruptions. Still primes well without Encumber due to the puncture and forced element mechanic.
Twin Graks are an incredible hybrid weapon. High single target damage so they shred easily, and absolutely enormous proc per second rate for priming.
Tenet Cycron with magnetic progenitor. Beam weapon so procs rapidly, and very high damage output. Can do mag/viral for priming. Magnetic rapidly shreds overguard. Mag beam so rapidly pops bubbles.
Lex Prime Incarnon. Has enough damage to spare that it can afford to use a utility arcane. Easily oneshots eximus, and the punchthrough and large projectile can deal with Ancient Healer overguard balls.
Tenet Plinx. Lex but radial instead of columnar.
Awaiting testing
Epitaph (waiting for Prime), has higher status and an even IPS spread, as well as two guaranteed elements before modding. Charge shot might be good enough for oneshotting Eximus overguard. Probably not enough damage on the radial tapshot for Archimedea AoE strip.
Synoid Gammacor Incarnon. If you're not running a Contagion Verglas robo buddy, has the nice utility of forced Cold procs. Does spicy AoE hits a la the Tenet Plinx. Un-carnon form has innate mag. The lovely space cubes restore your energy <3