I'll start with a brief history of skirts. Some of the details are fuzzy because teams were intentionally unclear about how they used skirts. Other errors are caused by my memory.
Fixed skirts were probably first used on the Chaparral and McLaren Group 7 sports cars. They were used longitudinally and laterally to create a low pressure area around/behind the driver's bottom, Chaparral using a fan. In F1 fixed skirts appeared with McLaren and Brabham in the mid 1970s, in the fashion of the McLaren Can Am cars and would have been in use at the time that Lotus were developing their wing and ground effects cars.
Lotus tested and abandoned a brush system (similar to a draught excluder or the seals on revolving doors) on the model 77 and 78. For the 78, Lotus also tested a fixed skirt with a hinge half way up. By the time that the Lotus 79 briefly dominated, Lotus had decided to adopt sliding skirts, arguing that they were fixed to the sidepod wall and thus were not a moveable aerodynamic entity... For the Lotus 80, the team used a sliding skirt which weaved around the rear wheel (i.e. not straight) which did not rise and fall correctly. Other teams (e.g. Ligier) experimented with straight sliding skirts in the rear wheel region (eventually banned). Sliding skirts were banned completely after a few years and minimum ride height regulations were introduced, leading to the use of fixed skirts with ride height adjusting suspension (Brabham followed by everyone else). A few more years later, the regs required 'flat bottomed' cars, leading to the diffuser, exhaust extraction and more.
* Brush skirts: Didn't seal sufficiently for late 1970s designs.
* All skirts: Low pressure could 'suck' the skirt underneath the sidepod. Lots of photos show skirts which are not hanging vertically. Trailing/running edge wear. Wear surfaces used ceramic or metal inserts which broke away creating track debris. Intermittent leaks in the venturi tunnels (caused by track bumps etc) lead to 'porpoising' -- ride height changes or pitching. Insufficient rigidity in the sidepod would have made everything unpredictable.
* Fixed skirts, with or without a hinge: Don't seal until the car is pushed down on its springs and tend to leak on anything but the smoothest track.
* Sliding skirts: Need to be straight and the rise/fall sliders require a lot of testing/development. Top teams ran a separate group to maintain skirts at the races, which increased costs. Riding over kerbs is a risky act and an extensive off-track excursion rips skirts off completely. In F1, skirt loss on one side of the car tended to result in a high speed spin -- although I am sure that there were even scarier accidents as well.
* Fixed skirts plus lowering suspension: Reduced one cost (skirt maintenance group) and introduced a new one (ride height hydraulics group). F1 teams and their suppliers had done an incredible job to create fixed skirts which worked as well as sliding skirts.
If sliding skirts were permitted in F1 today, thousands of spectators would die from boredom. The cars would have so much downforce that they would not budge from their line on anything but the slowest corner.
How much downforce did cars generate at any particular time? It's impossible to say owing to different testing facilities, but we can assess high speed corner performance (top speed through a known radius). In CART racing where venturi tunnels were permitted for a few years longer than F1, skirtless cars cornered at whopping Gs.