I still consider Lias Issodon one of the best Independent Characters in 40K, in particular due to the versatility of his special rules and wargear, and I think he pairs up especially well with the Deathwatch faction, which right now is struggling in the competitive meta due to its extreme “glass cannon” nature.
In particular, here are some things about Lias Issodon that mesh well with a Deathwatch force:
 “Master of Ambush” Warlord Trait — Being able to GUARANTEE infiltrating himself and three other non-vehicle units is a very big deal for Deathwatch, not only because they have limited mobility options and they cost a lot for even their base units (meaning it can be prohibitive to purchase lots of transports for them), but also because it allows them to maximize their close ranged shooting and assault abilities, which really are the Deathwatch’s main strengths. Lias Issodon allows you to potentially put up to three Kill Teams with max Frag Cannons, or three units of Terminators with max Assault Cannons, or three units of Combi-Grav Veterans, 18″ away from their target before the game even begins. Turn 1, you either move the units up 6″ (i.e. for Frag Cannons, Heavy Flamers, etc) or stay still and unleash full powered Grav Gun Salvos, thus maximizing your “alpha strike.” Significantly, if one of these units has a DW Independent Character in it (like an Aquila Kill Team with a Librarian), then you can equip him with the Beacon Angelis Relic and “summon” a fourth DW unit in close to concentrate your close ranged firepower even better turn 1.
 Shrouded USR and Locator Beacon — Since Issodon has Shrouded in his special rules, he confers it to any squad he joins. This means that he can attach to an allied Deathwatch unit and give it a 3+ cover save in even basic terrain (Note: this will make him lose his Chapter Tactics and the squad he is attached to lose its Mission Tactics, so there is a drawback to this). This obviously works well for a “fire base” unit like a squad of Veterans with Stalker Boltguns camping back in your DZ. Alternately, if you Infilitrate him along with a DW Biker squad, then even in the open they will get a 2++ Jink save (i.e. 4++ save from Jink + Shrouded), helping them survive until they can charge in on Turn 2 with their cheap PWs. Finally, since Lias Issodon has a Locator Beacon as part of his Wargear, he gives a DW army the ability to have two separate units for bringing in Drop Pods, Terminators, Jump Infantry, Kill Teams, etc. with no scatter (i.e. Lias Issodon can bring in one and a DW character with the Beacon Angelis can bring in another), thus increasing the flexibility and precision of a Deathwatch force, which is critical for them, given their low numbers and lack of durability.
 “Cunning Strategist” and “Infiltrate, Isolate, and Destroy” rules — The “Cunning Strategist” rule basically gives your forces free Reserve re-rolls and causes enemy Reserve rolls to have a -1 modifier, in turn giving you better control of your reinforcements and improving your chances of out-massing your opponent at the critical time and place on the battlefield, even if he has a bigger overall army than you (which is almost guaranteed when playing Deathwatch). As for “Infiltrate, Isolate, and Destroy,” this gives you small bonus chance to sabotage/weaken an enemy unit or vehicle before the game begins. While it is not necessarily a game changer, I once got lucky with this rule and “Haywired” to death a Necron Triarch Stalker before the game even began, which meant that my opponent did not get to boost the BS of his surrounding Necron forces the entire game, helping my cause significantly.
 Allied Support — Of course, you can only include Lias Issodon with a Deathwatch army by taking him in a Space Marine CAD or Allied Detachment alongside the main DW force. While this can reduce it the points you have to spend on DW units, it also opens up the possibility of bringing some very solid reinforcing units that synergize quite well with the enforcers of the Ordos Xenos. Examples include Space Marine Scouts in Land Speeder Storms (great as cheap Objective Seizers and harassment forces to take the pressure off your main DW squads), Drop Pod Centurions with Grav Cannons (to provide heavy ranged fire support against TEQs, MCs, etc.), Quad Mortar batteries (for a versatile, long-ranged fire base to mitigate enemy hordes and even deal with light vehicles), Ironclad and Siege Dreadnoughts (to soak up enemy attention, provide good close combat punch, and serve as a “shield” for nearby DW squads), and Bike equipped Chapter Masters/Chaplains/Librarians (to lead the mobile DW Vanguard and Bike squads, since currently no DW characters can take Space Marine Bikes and only a Watch Captain can take a Jump Pack).
Overall, while Lias Issodon is an excellent addition to any Imperium army, I think he combines particularly well with the forces of the Deathwatch and really bumps them up from “struggling” to at least the “moderately competitive” level in terms of competing with other armies, especially if you flesh it out with some of the supplemental Space Marine units noted above. Since they are all Astartes, from a fluff and painting perspective, you can easily represent Lias and his Space Marines as “counts as” Deathwatch in order to not break up the flow and visual presentation of a pure Deathwatch army.